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.

It should only be used for low-level things and with caution, for example
putting mounted VFS data in ROM or the static data section.

The vfs_fat_fsusermount test is no longer relevant so has been removed.

The VFS sub-system supports mounting of an arbitrary number of devices
(limited only by available RAM).  The internal flash is now mounted at
"/flash".

This patch includes a new module "uos_vfs" which can be used for testing
the VFS syb-system.

The VfsFat object can now be mounted by the generic VFS sub-system.

This provides mp_vfs_XXX functions (eg mount, open, listdir) which are
agnostic to the underlying filesystem type, and just require an object with
the relevant filesystem-like methods (eg .mount, .open, .listidr) which can
then be mounted.

These mp_vfs_XXX functions would typically be used by a port to implement
the "uos" module, and mp_vfs_open would be the builtin open function.

This feature is controlled by MICROPY_VFS, disabled by default.

And move declaration of mp_fat_vfs_type to this file.

The new version of FatFs requires a minimum of 50 blocks on the device.
Also, some tests no longer make sense with an OO vfs.

If MICROPY_VFS_FAT is enabled by a port then the port must switch to using
MICROPY_FATFS_OO.  Otherwise a port can continue to use the FatFs code
without any changes.

From https://github.com/micropython/oofatfs, branch work-R0.12b,
commit a346ccec123c2e4d887e2751d64156208d03bff4.

Previous to this patch the qemu-arm tests were compiled with is_relp=true
meaning that the __repl_print__ function was called for all lines of code
in the outer scope.  This is not the right behaviour for scripts that are
executed as though they were a file (eg tests).

With this fix the micropython/heapalloc_str.py test now works so it is
removed from the test blacklist.

Starts with concatenation with an empty string.

In this, don't allocate copy, just return non-empty string. This helps
with a standard pattern of buffering data in case of short reads:

    buf = b""
    while ...:
        s = f.read(...)
        buf += s
        ...

For a typical case when single read returns all data needed, there won't
be extra allocation. This optimization helps uasyncio.

They are one-line functions and having them inline in mp_init/mp_deinit
eliminates the overhead of a function call, and matches how other state
is initialised in mp_init.

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).

Fixes #2806.