This patch allows mounting of VFS objects right at the root directory, eg
os.mount(vfs, '/'). It still allows VFS's to be mounted at a path within
the root, eg os.mount(vfs, '/flash'), and such mount points will override
any paths within a VFS that is mounted at the root.
For example, if the current directory is the root dir then this patch
allows one to do uos.listdir('mnt'), where 'mnt' is a valid mount point.
Previous to this patch such a thing would not work, on needed to do
uos.listdir('/mnt') instead.
If the mounted object doesn't have a "mount" method then assume it's a
block device and try to detect the filesystem. Since we currently only
support FAT filesystems, the behaviour is to just try and create a VfsFat
object automatically, using the given block device.
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.
