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

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.

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).

With minor changes to adc.c and storage.c to support the F769.

In particular, this makes the L4 .isr_vector section 16K in size so it's
the same as the F4/F7 MCUs.  The patch also moves the L4 filesystem to
the end of flash, which allows for 512K filesystem on the 1Mb devices
like the STM32L476DISC.

The way to lookup the flash sector now uses a much simpler table for
all MCUs.

This patch adds support to fsusermount for multiple block devices
(instead of just one).  The maximum allowed is fixed at compile time by
the size of the fs_user_mount array accessed via MP_STATE_PORT, which
in turn is set by MICROPY_FATFS_VOLUMES.

With this patch, stmhal (which is still tightly coupled to fsusermount)
is also modified to support mounting multiple devices   And the flash and
SD card are now just two block devices that are mounted at start up if
they exist (and they have special native code to make them more
efficient).

You can now create (singleton) objects representing the flash and SD
card, using:

    flash = pyb.Flash()
    sdcard = pyb.SDCard()

These objects provide the block protocol.

USB serial is now working for F7.

Internal file storage is now working for F7.  The flash is laid out a bit
differently to the F4 - 4 x 32K, 1 x 128K with the rest 256K, so the
internal storage is 96K.

Added more pind definitions for STM32F7DISC board.  Made USART1 be the
default HWUART repl.  The STLINK usb connector also looks like a USB
serial port which is attached to USART1 on the STM32F7DISC.

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.

It uses a 16k cache buffer and so the filesystem size is limited.

It defines types used by all other headers.

Fixes #691.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Internal flash used for the filesystem is now written (from the cache)
only after a 5s delay, or when a file is closed, or when the drive is
unmounted from the host.  This delay means that multiple writes can
accumulate in the cache, and leads to less writes to the flash, making
it last longer.

It's implemented by a high-priority interrupt that takes care of flash
erase and write, and flushing the cache.

This is still only an interim solution for the flash filesystem.  It
eventually needs to be replaced with something that uses less RAM for
the cache, something that can use more of the flash, and something that
does proper wear levelling.

If SD card is present on (soft) reset then it's mounted on 1:/ and files
can be openned using, eg, open('1:/test.txt', 'r'), or 'w' for writing.

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

Use the storage cache not only for writing but also for reading. This
avoids reading stale data and thus data loss.

A big change.  Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object).  This scheme follows CPython.  Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.

Also change name prefix, from py_ to mp_ (mp for Micro Python).