Rename recently introduced "sync" method to "flush" for consistency with
usual files.

In 1.6, Zephyr switched to "unified kernel" and new API set. Older kernel
API is supported, but marked as deprecated and leads to warnings.

If you have longish operations on the db (such as logging data) it may
be desirable to periodically sync the database to the disk.  The added
btree.sync() method merely exposes the berkley __bt_sync function to the
user.

Select and poll will now work on socket objects.

Implementation of polling may need further fine tuning, but basic
functionality works (tested on esp8266).

To make moduselect be usable by any port.

The constants MP_IOCTL_POLL_xxx, which were stmhal-specific, are moved
from stmhal/pybioctl.h (now deleted) to py/stream.h.  And they are renamed
to MP_STREAM_POLL_xxx to be consistent with other such constants.

All uses of these constants have been updated.

If the destination of os.rename() exists then it will be overwritten if it
is a file.  This is the POSIX behaviour, which is also the CPython
behaviour, and so we follow suit.

See issue #2598 for discussion.

Add 2 macros in mphalport.h that clean and invalidate data caches only on
STM32F7 MCUs.  They are needed to ensure the cache coherency before/after
DMA transferts.

* MP_HAL_CLEANINVALIDATE_DCACHE cleans and invalidate the data cache. It
  must be called before starting a DMA transfer from the peripheral to the
  RAM memory.

* MP_HAL_CLEAN_DCACHE cleans the data cache.  It must be called before
  starting a DMA transfert from the RAM memory to the peripheral.

These macros are called in sdcard.c, before reading from and writing to
the SDCard, when DMA is used.

Fill is a very common operation (eg to clear the screen) and it is worth
optimising it, by providing a specialised fill_rect function for each
framebuffer format.

This patch improved the speed of fill by 10 times for a 16-bit display
with 160*128 pixels.

For backwards compatibility.  It simple creates a frame buffer with the
MVLSB format.

Rename FrameBuffer1 into FrameBuffer and make it handle different bit
depths via a method table that has getpixel and setpixel.  Currently
supported formats are MVLSB (monochrome, vertical, LSB) and RGB565.

Also add blit() and fill_rect() methods.

The temperature sensor on F4 and F7 MCUs is mostly, but not always, on
channel 16.  To retain compatibility across all these MCUs this patch
maps the user-facing channel 16 to the internal temperature sensor.

It now uses factory calibration values to get a more accurate value, and
returns a float instead of an integer.

By using factory calibration values.

That's where Zephyr keeps object files and produced executables.

The "mask" parameter is used to select which pins the ADCAll constructor
will initialise to analog mode.  It defaults to all pins (0xffffffff),
which is backwards compatible with previous behaviour.

Also make l4 config the default for STM32L476DISC board.

These were inadvertently removed with a recent upgrade to CMSIS, where
those registers were no longer defined in the CMSIS headers, and hence
no longer extracted.

All assemblers should "derive" from mp_asm_base_t.

Also add a test to check case when 3rd arg is 0.

And then it can run with the native emitter.

Short circuiting is handled correctly by c_if_cond, and constants within
short-circuit expressions are optimised by the parser.

Improves coverage because it tests the case where the arg does not have a
__len__ slot.

With this patch machine.I2C() uses HW I2C peripheral blocks on F4 MCUs.
Software I2C is used for other MCUs.