fs-test: Add FAT16 support

Currently we can only test FAT32 which is the default FAT version that
mkfs.vfat creates by default. Instead make it explicitly create either a
FAT16 or a FAT32 volume. This allows us to exercise more code, for
instance the root directory handling is done differently in FAT32 than
the older FATs.

Adding FAT12 support is a much bigger job since the test creates a 2.5GB
file and the FAT12 maximum partition size is way smaller than that.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Tom Rini <trini@konsulko.com>