Fix the first two FAT entries

The first entry (cluster 0) should have the 'FAT ID' in the lower
bits and the remaining bits of the entry (treated as 28-bits for FAT32)
set to 1.

The second entry should have all bits set (treated again as 28-bits
for FAT32).

Signed-off-by: David Scott <dave.scott@eu.citrix.com>