fs: descend into sub directories when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/fdos/Makefile b/fs/fdos/Makefile
index 95480af..2f8b5ad 100644
--- a/fs/fdos/Makefile
+++ b/fs/fdos/Makefile
@@ -10,4 +10,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_CMD_FDOS) := fat.o vfat.o dev.o fdos.o fs.o subdir.o
+obj-y := fat.o vfat.o dev.o fdos.o fs.o subdir.o