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This new function controls what happens on a hard-fault:
- debugging disabled: board will do a reset
- debugging enabled: board will print registers and stack and flash LEDs

The default is disabled, ie to do a reset.  This is different to previous
behaviour which flashed the LEDs and waited indefinitely.

This patch brings the _thread module to stmhal/pyboard.  There is a very
simple round-robin thread scheduler, which is disabled if there is only
one thread (for efficiency when threading is not used).

The scheduler currently switches threads at a rate of 250Hz using the
systick timer and the pend-SV interrupt.

The GIL is disabled so one must be careful to use lock objects to prevent
concurrent access of objects.

The threading is disabled by default, one can enabled it with the config
option MICROPY_PY_THREAD to test it out.

If the SD card is mounted then its libraries (ie those that are imported)
should override any in /flash.

The first partition is mounted as "/sd" and subsequent partitions are
mounted as "/sd<part_num>".  This is backwards compatible with the previous
behaviour, which just mounted the first partition on "/sd".

At this point, only FatFs filesystems are mounted.

The stack pointer should start pointing 1 byte past the top of the end of
RAM.

stmhal has MULTI_PARTITION enabled for FatFs and so these values need to be
initialised.

vfs.h is for generic VFS declarations, and vfs_fat.h is for VfsFat
specific things.

Everyone should now be using the new ooFatFs library.  The old one is no
longer supported and will be removed.

This patch makes the following configuration changes:
- MICROPY_FSUSERMOUNT is disabled, removing old mounting infrastructure
- MICROPY_VFS is enabled, giving new VFS sub-system
- MICROPY_VFS_FAT is enabled, giving uos.VfsFat type
- MICROPY_FATFS_OO is enabled, to use new ooFatFs lib, R0.12b

User facing API should be almost unchanged.  Most notable changes are
removal of os.mkfs (use os.VfsFat.mkfs instead) and pyb.mount doesn't
allow unmounting by passing None as the device.

And move declaration of mp_fat_vfs_type to this file.

The order now follows that in py/mpconfig.h and is a bit cleaner and easier
to maintain.  No options were changed/added/removed with this patch, it's
just a reordering.

To use this feature a port should define MICROPY_HW_SPIFLASH_SIZE_BITS
along with x_CS, x_SCK, x_MOSI, x_MISO (x=MICROPY_HW_SPIFLASH).  This will
then use external SPI flash on those pins instead of the internal flash.

The SPI is done using the software implementation.  There is currently only
support for standard SPI (ie not dual or quad mode).

stmhal will now be built by default with frozen bytecode from scripts
stored in the stmhal/modules/ directory.  This can be disabled or
changed to another directory by overridding the make variable
FROZEN_MPY_DIR.

This implements flow control in case user does not call recv method often
enough (it tells host side to stop sending more data).

Otherwise, they serve reoccurring source of copy-paste mistakes and
breaking nanbox build.

Sys-tick resolution is 1ms and a value of 2 will give a delay between 1ms
and 2ms (whereas a value of 1 gives a delay between 0ms and 1ms, which is
too short).

The HAL_UART_Transmit function has changed in the latest HAL version such
that the Timeout is a timeout for the entire function, rather than a
timeout between characters as it was before.  The HAL function also does
not allow one to reliably tell how many characters were sent before the
timeout (if a timeout occurred).

This patch provides a custom function to do UART transmission, completely
replacing the HAL version, to fix the above-mentioned issues.