kernel: Allow overriding dtbo target name

gs101 kernel uses google/dtbo.img target for dtbo.img

Change-Id: Ib03f35c0035f56d303771de9399bcce3d02c0775
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 55387e7..5ea7962 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -466,7 +466,7 @@
 	$(MKDTBOIMG) create $@ --page_size=$(BOARD_KERNEL_PAGESIZE) $(shell find $(DTBO_OUT)/arch/$(KERNEL_ARCH)/boot/dts -type f -name "*.dtbo" | sort)
 endif # BOARD_DTBO_CFG
 else
-	$(call make-dtbo-target,dtbo.img)
+	$(call make-dtbo-target,$(TARGET_KERNEL_DTBO))
 endif # BOARD_KERNEL_SEPARATED_DTBO
 	$(hide) touch -c $(DTBO_OUT)
 endif # BOARD_CUSTOM_DTBOIMG_MK