fs: fat: support unlink

In this patch, unlink support is added to FAT file system.
A directory can be deleted only if it is empty.

In this implementation, only a directory entry for a short file name
will be removed. So entries for a long file name can and should be
reclaimed with fsck.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>