Fix FAT signature so that u-boot works
Fix FAT signature so that u-boot works

U-boot requires either FAT12 or FAT16 instead of the generic "FAT"

fhunleth/fwup,michaelkschmidt/fwup,edahlseng/fwup,fhunleth/fwup,michaelkschmidt/fwup,michaelkschmidt/fwup,edahlseng/fwup,fhunleth/fwup