kernel: Don't re-define dtbo.img names

These are already set in BoardConfig, no need to set them again

Change-Id: I30eb44f748d0f427145beb5d6cd53ece14281abd
diff --git a/build/tasks/dtbo.mk b/build/tasks/dtbo.mk
index 674eae0..cbe1214 100644
--- a/build/tasks/dtbo.mk
+++ b/build/tasks/dtbo.mk
@@ -3,8 +3,6 @@
 
 MKDTIMG := $(HOST_OUT_EXECUTABLES)/mkdtimg$(HOST_EXECUTABLE_SUFFIX)
 
-BOARD_PREBUILT_DTBOIMAGE := $(PRODUCT_OUT)/dtbo-pre.img
-
 # Most specific paths must come first in possible_dtbo_dirs
 possible_dtbo_dirs = $(KERNEL_OUT)/arch/$(KERNEL_ARCH)/boot/dts $(KERNEL_OUT)/arch/arm/boot/dts
 
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index a2bb3f4..0fdbba3 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -270,7 +270,6 @@
 		 $(call make-kernel-target,alldefconfig)
 
 ifeq ($(TARGET_NEEDS_DTBOIMAGE),true)
-BOARD_PREBUILT_DTBOIMAGE = $(PRODUCT_OUT)/dtbo/arch/$(KERNEL_ARCH)/boot/dtbo.img
 $(BOARD_PREBUILT_DTBOIMAGE):
 	echo -e ${CL_GRN}"Building DTBO.img"${CL_RST}
 	$(call make-dtbo-target,$(KERNEL_DEFCONFIG))