The specific board can be selected with the BOARD makefile variable.  This
defaults (if not specified) to BOARD=GENERIC, which is the original default
firmware build.  For the 512k target use BOARD=GENERIC_512K.

Fixes issue #5162.

Enable in board config with: #define MBOOT_USB_RESET_ON_DISCONNECT (1)

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 BLE stack is not performance critical, so put it in external
memory-mapped flash to save internal flash for other things (like frozen
bytecode).

Fixes regression introduced by 6705767d

To test build support with Python 2.7.

Selectable via the command line: -march=xtensawin.

Enabled via the configuration MICROPY_EMIT_XTENSAWIN.

Window-specific asm emit functions are added, along with a new macro option
GENERIC_ASM_API_WIN.

In which case place the native function prelude in a bytes object, linked
from the const_table of that function.  An architecture should define
N_PRELUDE_AS_BYTES_OBJ to 1 before including py/emitnative.c to emit
correct machine code, then enable MICROPY_EMIT_NATIVE_PRELUDE_AS_BYTES_OBJ
so the runtime can correctly handle the prelude being in a bytes object.

To enable this feature the N_NLR_SETJMP macro should be set to 1 before
including py/emitnative.c.

Such that args/return regs for the parent are different to args/return regs
for child calls.  For an architecture to use this feature it should define
the REG_PARENT_xxx macros before including py/emitnative.c.

This test was found by missing coverage of a branch in py/nativeglue.c.

Commit 3f6ffe05 implemented PEP479 but did
not catch the case fixed in this commit.  Found by coverage analysis, that
the VM had uncovered code.

Prior to this commit, when unwinding through an active finally the stack
was not being correctly popped/folded, which resulting in the VM crashing
for complicated unwinding of nested finallys.

This should be fixed with this commit, and more tests for return/break/
continue within a finally have been added to exercise this.

STM32F0 has PCLK=48MHz and maximum ADC clock is 14MHz so use PCLK/4=12MHz
to stay within spec of the ADC peripheral.  In pyb.ADC set common sampling
time to approx 4uS for internal and external sources.  In machine.ADC
reduce sample time to approx 1uS for external source, leave internal at
maximum sampling time.

As of 7d58a197, `NULL` should no longer be
here because it's allowed (MP_QSTRnull took its place).  This entry was
preventing the use of MP_QSTR_NULL to mean "NULL" (although this is not
currently used).

A blacklist should not be needed because it should be possible to intern
all strings.

Fixes issue #5140.

This check follows CPython's behaviour, because 'import *' always populates
the globals with the imported names, not locals.

Since it's safe to do this (doesn't lead to a crash or undefined behaviour)
the check is only enabled for MICROPY_CPYTHON_COMPAT.

Fixes issue #5121.

The previous commit changed the default configuration on L4 MCUs to use
LSI, so configure these boards to use LSE again.

This commit adds the option to use HSE or MSI system clock, and LSE or LSI
RTC clock, on L4 MCUs.

Note that prior to this commit the default clocks on an L4 part were MSI
and LSE.  The defaults are now MSI and LSI.

In mpconfigboard.h select the clock source via:

    #define MICROPY_HW_RTC_USE_LSE (0) or (1)
    #define MICROPY_HW_CLK_USE_HSE (0) or (1)

and the PLLSAI1 N,P,Q,R settings:

    #define MICROPY_HW_CLK_PLLSAIN (12)
    #define MICROPY_HW_CLK_PLLSAIP (RCC_PLLP_DIV7)
    #define MICROPY_HW_CLK_PLLSAIQ (RCC_PLLQ_DIV2)
    #define MICROPY_HW_CLK_PLLSAIR (RCC_PLLR_DIV2)

In case of LED1 being present, do a short blink during startup
instead of turning it on and leave it on.

Aligned implementation with the STM32 port.
Added empty functions to be used when no LED is available.

The the nrfx driver is aware of chip specific registers, while
the raw HAL abstraction is not. This driver enables use of NVMC
in non-secure domain for nrf9160.

This patch moves the check for MICROPY_PY_MACHINE_TEMP to come
before the inclusion of nrf_temp.h. The nrf_temp.h depends on
the NRF_TEMP_Type which might not be defined for all nRF devices.

Tests correct decoding of the prelude to get the function name.

These are now only needed when persistent code is disabled.

This patch compresses the second part of the bytecode prelude which
contains the source file name, function name, source-line-number mapping
and cell closure information.  This part of the prelude now begins with a
single varible length unsigned integer which encodes 2 numbers, being the
byte-size of the following 2 sections in the header: the "source info
section" and the "closure section".  After decoding this variable unsigned
integer it's possible to skip over one or both of these sections very
easily.

This scheme saves about 2 bytes for most functions compared to the original
format: one in the case that there are no closure cells, and one because
padding was eliminated.

The start of the bytecode prelude contains 6 numbers telling the amount of
stack needed for the Python values and exceptions, and the signature of the
function.  Prior to this patch these numbers were all encoded one after the
other (2x variable unsigned integers, then 4x bytes), but using so many
bytes is unnecessary.

An entropy analysis of around 150,000 bytecode functions from the CPython
standard library showed that the optimal Shannon coding would need about
7.1 bits on average to encode these 6 numbers, compared to the existing 48
bits.

This patch attempts to get close to this optimal value by packing the 6
numbers into a single, varible-length unsigned integer via bit-wise
interleaving.  The interleaving scheme is chosen to minimise the average
number of bytes needed, and at the same time keep the scheme simple enough
so it can be implemented without too much overhead in code size or speed.
The scheme requires about 10.5 bits on average to store the 6 numbers.

As a result most functions which originally took 6 bytes to encode these 6
numbers now need only 1 byte (in 80% of cases).

This value is used often enough that it is better to cache it instead of
decode it each time.

Change from a pointer to an index, to make space in mp_code_state_t.

It was made obsolete in 6f9e3ff7