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/spear/common/Makefile b/board/spear/common/Makefile
index 08dc09f..b0ba320 100644
--- a/board/spear/common/Makefile
+++ b/board/spear/common/Makefile
@@ -5,7 +5,10 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-ifndef CONFIG_SPL_BUILD
+ifdef CONFIG_SPL_BUILD
+# necessary to create built-in.o
+obj- := __dummy__.o
+else
 obj-y	:= spr_misc.o
 obj-y	+= spr_lowlevel_init.o
 endif