changes include:
* use single GPIO.BSRR instead of BSRRH and BSRRL
* change HSE_STARTUP_TIMEOUT to 100 ms
* define LSE_STARTUP_TIMEOUT to 5 s

If a port defines MICROPY_READER_POSIX or MICROPY_READER_FATFS then
lexer.c now provides an implementation of mp_lexer_new_from_file using
the mp_reader_new_file function.

Implementations of persistent-code reader are provided for POSIX systems
and systems using FatFS.  Macros to use these are MICROPY_READER_POSIX and
MICROPY_READER_FATFS respectively.  If an alternative implementation is
needed then a port can define the function mp_reader_new_file.

Also, remove an "only" directive in u-modules description.

Adds about 200 bytes to the code size when constant folding is enabled.

It is split into 2 functions, one to make small ints and the other to make
a non-small-int leaf node.  This reduces code size by 32 bytes on
bare-arm, 64 bytes on unix (x64-64) and 144 bytes on stmhal.

Per the latest HW API, "SPI" class implements only master side of the
protocol, so mode=SPI.MASTER (which was static for WiPy anyway) is not
required (or allowed). This change is required to correspond to updated
documentation of machine.SPI class which no longer lists "mode".

The Mac version of find doesn't support -printf, so this changes
things to use sed to strip off the leading path element instead.

This includes StopIteration and thus are important to make Python-coded
iterables work with yield from/await.

Exceptions in Python send() are still not handled and left for future
consideration and optimization.

We allow 'exc.__traceback__ = None' assignment as a low-level optimization
of pre-allocating exception instance and raising it repeatedly - this
avoids memory allocation during raise. However, uPy will keep adding
traceback entries to such exception instance, so before throwing it,
traceback should be cleared like above.

'exc.__traceback__ = None' syntax is CPython compatible. However, unlike
it, reading that attribute or setting it to any other value is not
supported (and not intended to be supported, again, the only reason for
adding this feature is to allow zero-memalloc exception raising).

Its addition was due to an early exploration on how to add CPython-like
stream interface. It's clear that it's not needed and just takes up
bytes in all ports.

As required for further elaboration of uasyncio, like supporting baremetal
systems with wraparound timesources. This is not intended to be public
interface, and likely will be further refactored in the future.

Requires BUTTON defined in hwconfig, so far tested on DragonBoard 410c.

This way the caller doesn't need to initialise a big GPIO_InitTypeDef
struct, and HAL_GPIO_Init is no longer called.