fs: descend into sub directories when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/Makefile b/fs/Makefile
index ee01a7d..34dc035 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -11,15 +11,15 @@
 else
 obj-y				+= fs.o
 
-obj-y += cbfs/
-obj-y += cramfs/
-obj-y += ext4/
+obj-$(CONFIG_CMD_CBFS) += cbfs/
+obj-$(CONFIG_CMD_CRAMFS) += cramfs/
+obj-$(CONFIG_FS_EXT4) += ext4/
 obj-y += fat/
-obj-y += fdos/
-obj-y += jffs2/
-obj-y += reiserfs/
-obj-y += sandbox/
-obj-y += ubifs/
-obj-y += yaffs2/
-obj-y += zfs/
+obj-$(CONFIG_CMD_FDOS) += fdos/
+obj-$(CONFIG_CMD_JFFS2) += jffs2/
+obj-$(CONFIG_CMD_REISER) += reiserfs/
+obj-$(CONFIG_SANDBOX) += sandbox/
+obj-$(CONFIG_CMD_UBIFS) += ubifs/
+obj-$(CONFIG_YAFFS2) += yaffs2/
+obj-$(CONFIG_CMD_ZFS) += zfs/
 endif