vendor: build: add flag to skip building DTBO.img

some targets build the image with a custom recipe
avoid overwriting the recipe with the one in kernel.mk by checking the flag TARGET_PROVIDES_DTBOIMAGE

Change-Id: I33514233f7866deb91a7bf4ffe0a639711332e99
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 6ff7b3c..da5c40e 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -397,6 +397,7 @@
 
 endif # FULL_KERNEL_BUILD
 
+ifneq ($(TARGET_PROVIDES_DTBOIMAGE), true)
 TARGET_PREBUILT_DTBO = $(PRODUCT_OUT)/dtbo/arch/$(KERNEL_ARCH)/boot/dtbo.img
 $(TARGET_PREBUILT_DTBO): $(AVBTOOL)
 	echo -e ${CL_GRN}"Building DTBO.img"${CL_RST}
@@ -407,6 +408,7 @@
 		--partition_size $(BOARD_DTBOIMG_PARTITION_SIZE) \
 		--partition_name dtbo $(INTERNAL_AVB_DTBO_SIGNING_ARGS) \
 		$(BOARD_AVB_DTBO_ADD_HASH_FOOTER_ARGS)
+endif # TARGET_PROVIDES_DTBOIMAGE
 
 ## Install it