spl: Convert fat to spl_load

This converts the fat loader to use spl_load. Some platforms are very
tight on space, so we take care to only include the code we really need.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/spl_load.h b/include/spl_load.h
index 65aa6bb..5e0460d 100644
--- a/include/spl_load.h
+++ b/include/spl_load.h
@@ -96,6 +96,7 @@
  */
 #define SPL_LOAD_USERS \
 	IS_ENABLED(CONFIG_SPL_FS_EXT4) + \
+	IS_ENABLED(CONFIG_SPL_FS_FAT) + \
 	0
 
 #if SPL_LOAD_USERS > 1