fs: descend into sub directories when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/cbfs/Makefile b/fs/cbfs/Makefile
index 6f33d28..a106e05 100644
--- a/fs/cbfs/Makefile
+++ b/fs/cbfs/Makefile
@@ -3,4 +3,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_CMD_CBFS)	:= cbfs.o
+obj-y	:= cbfs.o