kbuild: add dummy obj-y to create built-in.o

We are going to switch over to Kbuild in upcoming commits.

Each makefile must have non-empty obj- or obj-y
to generate built-in.o on Kbuild.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/board/samsung/origen/Makefile b/board/samsung/origen/Makefile
index 37acba7..1add9fe 100644
--- a/board/samsung/origen/Makefile
+++ b/board/samsung/origen/Makefile
@@ -5,6 +5,9 @@
 #
 
 ifdef CONFIG_SPL_BUILD
+# necessary to create built-in.o
+obj- := __dummy__.o
+
 hostprogs-y := tools/mkorigenspl
 always := $(hostprogs-y)