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/freescale/common/Makefile b/board/freescale/common/Makefile
index 25f063d..f6a0879 100644
--- a/board/freescale/common/Makefile
+++ b/board/freescale/common/Makefile
@@ -13,7 +13,10 @@
 endif
 endif
 
-ifndef MINIMAL
+ifdef MINIMAL
+# necessary to create built-in.o
+obj- := __dummy__.o
+else
 obj-$(CONFIG_FSL_CADMUS)	+= cadmus.o
 obj-$(CONFIG_FSL_VIA)		+= cds_via.o
 obj-$(CONFIG_FMAN_ENET)	+= fman.o