FAT: implement fat_set_blk_dev(), convert cmd_fat.c

FAT: implement fat_set_blk_dev(), convert cmd_fat.c

This makes the FAT filesystem API more consistent with other block-based
filesystems. If in the future standard multi-filesystem commands such as
"ls" or "load" are implemented, having FAT work the same way as other
filesystems will be necessary.

Convert cmd_fat.c to the new API, so the code looks more like other files
implementing the same commands for other filesystems.

Signed-off-by: Stephen Warren <[email protected]>
Reviewed-by: Benoît Thébaudeau <[email protected]>