kernel: Allow building dtbo.img

taimen uses the dtbo.img as an DTB Overlay in order to use the same defconfig for both, taimen and wahoo.
The dtbo.img is built with "make dtbo.img". Allow automatically building it when setting TARGET_NEEDS_DTBOIMAGE to true, or by manually calling make dtboimage.

Thanks to luca020400 for help with the kernel.mk logic

[raymanfx] Add AVB hash footer support

Change-Id: Iaddafaa98e3e84880462a2c35b899e3fea21e161

build/tasks/kernel.mk

@@ -396,6 +396,17 @@ alldefconfig: $(KERNEL_OUT_STAMP)
 
 endif # FULL_KERNEL_BUILD
 
+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}
+	$(MAKE) $(MAKE_FLAGS) -C $(KERNEL_SRC) O=$(PRODUCT_OUT)/dtbo ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) $(KERNEL_CLANG_TRIPLE) $(KERNEL_CC) $(KERNEL_DEFCONFIG)
+	$(MAKE) $(MAKE_FLAGS) -C $(KERNEL_SRC) O=$(PRODUCT_OUT)/dtbo ARCH=$(KERNEL_ARCH) $(KERNEL_CROSS_COMPILE) $(KERNEL_CLANG_TRIPLE) $(KERNEL_CC) dtbo.img
+	$(AVBTOOL) add_hash_footer \
+		--image $@ \
+		--partition_size $(BOARD_DTBOIMG_PARTITION_SIZE) \
+		--partition_name dtbo $(INTERNAL_AVB_DTBO_SIGNING_ARGS) \
+		$(BOARD_AVB_DTBO_ADD_HASH_FOOTER_ARGS)
+
 ## Install it
 
 ifeq ($(NEEDS_KERNEL_COPY),true)
@@ -407,7 +418,19 @@ $(file) : $(KERNEL_BIN) | $(ACP)
 ALL_PREBUILT += $(INSTALLED_KERNEL_TARGET)
 endif
 
+ifeq ($(TARGET_NEEDS_DTBOIMAGE),true)
+file := $(INSTALLED_DTBOIMAGE_TARGET)
+ALL_PREBUILT += $(file)
+$(file) : $(TARGET_PREBUILT_DTBO) | $(ACP)
+	$(transform-prebuilt-to-target)
+
+ALL_PREBUILT += $(INSTALLED_DTBOIMAGE_TARGET)
+endif
+
 .PHONY: kernel
 kernel: $(INSTALLED_KERNEL_TARGET)
 
+.PHONY: dtbo
+dtbo: $(INSTALLED_DTBOIMAGE_TARGET)
+
 endif # TARGET_NO_KERNEL