fs: descend into sub directories when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/sandbox/Makefile b/fs/sandbox/Makefile
index faa7c16..ca238f6 100644
--- a/fs/sandbox/Makefile
+++ b/fs/sandbox/Makefile
@@ -10,4 +10,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_SANDBOX) := sandboxfs.o
+obj-y := sandboxfs.o