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 <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/include/fat.h b/include/fat.h
index cc85b06..706cd7a 100644
--- a/include/fat.h
+++ b/include/fat.h
@@ -212,6 +212,7 @@
 		      unsigned long maxsize);
 long file_fat_read(const char *filename, void *buffer, unsigned long maxsize);
 const char *file_getfsname(int idx);
+int fat_set_blk_dev(block_dev_desc_t *rbdd, disk_partition_t *info);
 int fat_register_device(block_dev_desc_t *dev_desc, int part_no);
 
 int file_fat_write(const char *filename, void *buffer, unsigned long maxsize);