sandbox: add FAT to the list of usable env drivers

Add the FAT environment driver to the priority list.

When testing the UEFI sub-system the EFI system partition is formatted with
FAT so it is reasonable to store the environment there.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/board/sandbox/sandbox.c b/board/sandbox/sandbox.c
index 3c63d45..902b99e 100644
--- a/board/sandbox/sandbox.c
+++ b/board/sandbox/sandbox.c
@@ -48,6 +48,7 @@
 static enum env_location env_locations[] = {
 	ENVL_NOWHERE,
 	ENVL_EXT4,
+	ENVL_FAT,
 };
 
 enum env_location env_get_location(enum env_operation op, int prio)