subsys/fs: Make FAT FS formatting code optional

The commit adds FS_FAFTFS_MKFS Kconfig option; the option, y by default,
allows to include mkfs supporting code, within fs_mount callback,
for ELM FAT file system driver.

When formatting of FAT FS volumes is not desired, this option may be
deselected to slightly reduce code size.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>