Should fix issue #1393.

In particular this fixes a bug where pyb.sync (and os.sync) fail because
they try to sync the user mounted device even if it's not mounted.

Disabled by default.  Adds 108 bytes to Thumb2 arch when enabled.

This allows a user-specified Timer for the triggering of the ADC read,
mirroring the new behaviour of DAC.write_timed.

Addresses issue #1129.

This allows the DAC to use a user-specified Timer for the triggering
(instead of the default Timer(6)), while still supporting original
behaviour.

Addresses issues #1129 and #1388.

This prevented pin_find_af* functions from being able to find some
of the alternate functions in the pin struct

Only those files which are needed by the stmhal port are added.

Also includes a dummy file (stm32f2xx_hal_pcd_ex.c) to keep the build
system the same for f4 and f2 MCU series.

This is in preparation for supporting other MCU series, such as
STM32F2xx.  Directory structure for the HAL is now hal/f4/{inc,src},
where "f4" will in the future be different for other series.

HAL source/header files that are not use are removed to reduce the size
of the code.

When enabled this allows the internal storage to be split over 2
contiguous regions of flash (two segments), and so the storage can be
increased.

This option is disabled by default, giving original behaviour.

This removes hard-coded DMA init params from dma_init(), instead defining
these parameters in a DMA_InitTypeDef struct that gets passed as an
argument to dma_init()
This makes dma_init more generic so it can be used for I2S and SD Card,
which require different initialization parameters.

To make it the same as SPI and UART.

This is so that the DMA can be shared by multiple peripherals.

If IRQs are disabled then the USB CDC buffer will never be
drained/filled and the sys-tick timer will never increase, so we should
not busy wait in this case.

Addresses issue #1268.

I tested this on my CERB40 board and it seems to be working fine.

Now all stmhal-based boards can name their peripherals (if they want) to
any string.

It's configurable and only enabled for stmhal port.

Fixes sign-compare warning.

Addresses issue #1255.

mp_obj_get_int_truncated will raise a TypeError if the argument is not
an integral type.  Use mp_obj_int_get_truncated only when you know the
argument is a small or big int.

esp8266 port now has working raw and friendly REPL, as well as working
soft reset (CTRL-D at REPL, or raise SystemExit).

tools/pyboard.py now works with esp8266 port.