This function is not used by the core but having it as part of the build
allows it to be used by user C modules, or board extensions.  The linker
won't include it in the final firmware if it remains unused.

Work done in collaboration with Jim Mussared aka @jimmo.

This makes a cleaner separation between the: driver, HCI UART and BT stack.
Also updated the naming to be more consistent (mp_bluetooth_hci_*).

Work done in collaboration with Jim Mussared aka @jimmo.

Move extmod/modbluetooth_nimble.* to extmod/nimble.  And move common
Makefile lines to extmod/nimble/nimble.mk (which was previously only used
by stm32).  This allows (upcoming) btstack to follow a similar structure.

Work done in collaboration with Jim Mussared aka @jimmo.

This commit improves pllvalues.py to generate PLL values for H7 MCUs that
are valid (VCO in and out are in range) and extend for the entire range of
SYSCLK values up to 400MHz (up to 480MHz is currently unsupported).

Most stm32 boards can now be built in nan-boxing mode via:

    $ make NANBOX=1

Note that if float is enabled then it will be forced to double-precision.
Also, native emitters will be disabled.

Because in network_wiznet5k the TX/RX buffers are set to 16k each when in
MACRAW mode, which is used with lwIP.

This commit removes the Makefile-level MICROPY_FATFS config and moves the
MICROPY_VFS_FAT config to the Makefile level to replace it.  It also moves
the include of the oofatfs source files in the build from each port to a
central place in extmod/extmod.mk.

For a port to enabled VFS FAT support it should now set MICROPY_VFS_FAT=1
at the level of the Makefile.  This will include the relevant oofatfs files
in the build and set MICROPY_VFS_FAT=1 at the C (preprocessor) level.

This requires a BLE wireless stack firmware to be already programmed in the
secure flash area of the device.

This commit adds an implementation of machine.Timer backed by the soft
timer mechanism.  It allows an arbitrary number of timers with 1ms
resolution, with an associated Python callback.  The Python-level API
matches existing ports that have a soft timer, and is used as:

    from machine import Timer
    t = Timer(freq=10, callback=lambda t:print(t))
    ...
    t = Timer(mode=Timer.ONE_SHOT, period=2000, callback=lambda t:print(t))
    ...
    t.deinit()

This commit adds an implementation of a "software timer" with a 1ms
resolution, using SysTick.  It allows unlimited number of concurrent
timers (limited only by memory needed for each timer entry).  They can be
one-shot or periodic, and associated with a Python callback.

There is a very small overhead added to the SysTick IRQ, which could be
further optimised in the future, eg by patching SysTick_Handler code
dynamically.

Not enough to detect f7/h7, need to use the specific parts.

Follow-up to 580a2656.

All symlinks are removed, frozen files are now referenced via
boards/manifest.py.

Identical to cd527bb3 but for doubles.
This gives a -2.754% improvement on bm_float.py, and -35% improvement on
calling sqrt in a loop.

On other ports (e.g. ESP32) they provide a complete Nimble implementation
(i.e. we don't need to use the code in extmod/nimble). This change
extracts out the bits that we don't need to use in other ports:
 - malloc/free/realloc for Nimble memory.
 - pendsv poll handler
 - depowering the cywbt

Also cleans up the root pointer management.

The new fdcan.c file provides the low-level C interface to the FDCAN
peripheral, and pyb_can.c is updated to support both traditional CAN and
FDCAN, depending on the MCU being compiled for.

can.c now contains the low-level C interface to the CAN peripheral, and
pyb_can.c the Python-level class/methods/constants.

This new series of MCUs is similar to the L4 series with an additional
Cortex-M0 coprocessor.  The firmware for the wireless stack must be managed
separately and MicroPython does not currently interface to it.  Supported
features so far include: RTC, UART, USB, internal flash filesystem.

By using "MICROPY_FLOAT_IMPL = none" in its mpconfigboard.mk file.

stm32lib now provides system_stm32XXxx.c source files for all MCU variants,
which includes SystemInit and prescaler tables.  Since these are quite
standard and don't need to be changed, switch to use them instead of custom
variants, making the start-up code cleaner.

The SystemInit code in stm32lib was checked and is equivalent to what is
removed from the stm32 port in this commit.

This means that each MCU now gets a unique IRQ table, eg a specific one for
STM32F405, STM32F411, etc rather than just STM32F4xx.

This HAL file is now patched so it doesn't have these warnings.

Entering a bootloader (ST system bootloader, or custom mboot) from software
by directly branching to it is not reliable, and the reliability of it
working can depend on the peripherals that were enabled by the application
code.  It's also not possible to branch to a bootloader if the WDT is
enabled (unless the bootloader has specific provisions to feed the WDT).

This patch changes the way a bootloader is entered from software by first
doing a complete system reset, then branching to the desired bootloader
early on in the start-up process.  The top two words of RAM (of the stack)
are reserved to store flags indicating that the bootloader should be
entered after a reset.

Eventually these responses could be filled in by a function to make their
contents dynamic, depending on the attached logical units.  But for now
they are fixed, and this patch fixes the MODE SENSE(6) responses so it is
the correct length with the correct header.

SCSI can support multiple logical units over the one interface (in this
case over USBD MSC) and here the MSC code is reworked to support this
feature.  At this point only one LU is used and the behaviour is mostly
unchanged from before, except the INQUIRY result is different (it will
report "Flash" for both flash and SD card).

To use it a board should define MICROPY_PY_USSL=1 and MICROPY_SSL_MBEDTLS=1
at the Makefile level.  With the provided configuration it adds about 64k
to the build.

Enable it by setting MICROPY_PY_NETWORK_CYW43=1 at the Makefile level.

The new function factory_reset_make_files() populates the given filesystem
with the default factory files.  It is defined with weak linkage so it can
be overridden by a board.

This commit also brings some minor user-facing changes:

- boot.py is now no longer created unconditionally if it doesn't exist, it
  is now only created when the filesystem is formatted and the other files
  are populated (so, before, if the user deleted boot.py it would be
  recreated at next boot; now it won't be).

- pybcdc.inf and README.txt are only created if the board has USB, because
  they only really make sense if the filesystem is exposed via USB.

This patch makes the DAC driver simpler and removes the need for the ST
HAL.  As part of it, new helper functions are added to the DMA driver,
which also use direct register access instead of the ST HAL.

Main changes to the DAC interface are:

- The DAC uPy object is no longer allocated dynamically on the heap,
  rather it's statically allocated and the same object is retrieved for
  subsequent uses of pyb.DAC(<id>).  This allows to access the DAC objects
  without resetting the DAC peripheral.  It also means that the DAC is only
  reset if explicitly passed initialisation parameters, like "bits" or
  "buffering".

- The DAC.noise() and DAC.triangle() methods now output a signal which is
  full scale (previously it was a fraction of the full output voltage).

- The DAC.write_timed() method is fixed so that it continues in the
  background when another peripheral (eg SPI) uses the DMA (previously the
  DAC would stop if another peripheral finished with the DMA and shut the
  DMA peripheral off completely).

Based on the above, the following backwards incompatibilities are
introduced:

- pyb.DAC(id) will now only reset the DAC the first time it is called,
  whereas previously each call to create a DAC object would reset the DAC.
  To get the old behaviour pass the bits parameter like: pyb.DAC(id, bits).

- DAC.noise() and DAC.triangle() are now full scale.  To get previous
  behaviour (to change the amplitude and offset) write to the DAC_CR (MAMP
  bits) and DAC_DHR12Rx registers manually.

Enable it via MICROPY_HW_ENABLE_MMCARD.