If CONFIG_USE_BUILTIN_DTB is enabled, but CONFIG_BUILTIN_DTB_SOURCE
is empty (for example, allmodconfig), a build error occurs.

Surround obj-y with ifneq ... endif.

I replaced $(CONFIG_USE_BUILTIN_DTB) with 'y' since this is always
the case from the following code from arch/sh/Makefile:

  core-$(CONFIG_USE_BUILTIN_DTB)  += arch/sh/boot/dts/

Signed-off-by: Masahiro Yamada <redacted>
---

 arch/sh/boot/dts/Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/sh/boot/dts/Makefile b/arch/sh/boot/dts/Makefile
index 715def0..01d0f7f 100644
--- a/arch/sh/boot/dts/Makefile
+++ b/arch/sh/boot/dts/Makefile

@@ -1 +1,3 @@
-obj-$(CONFIG_USE_BUILTIN_DTB) += $(patsubst "%",%,$(CONFIG_BUILTIN_DTB_SOURCE)).dtb.o
+ifneq ($(CONFIG_BUILTIN_DTB_SOURCE),"")
+obj-y += $(patsubst "%",%,$(CONFIG_BUILTIN_DTB_SOURCE)).dtb.o
+endif

-- 
2.7.4