vendor: kernel: Add back support for custom dtboimg makefile

It was removed accidentally in commit fe00ea9.

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>
Change-Id: Icadd7b17b3a732fc2c8aa14f1e4dbd4bc66eb7f9

build/tasks/kernel.mk

@@ -43,6 +43,8 @@
 #
 #   BOARD_DTBO_CFG                     = Path to a mkdtboimg.py config file
 #
+#   BOARD_CUSTOM_DTBOIMG_MK            = Path to a custom dtboimage makefile
+#
 #   KERNEL_CC                          = The C Compiler used. This is automatically set based
 #                                          on whether the clang version is set, optional.
 #   KERNEL_LD                          = The Linker used. This is automatically set based
@@ -297,6 +299,9 @@ alldefconfig: $(KERNEL_OUT)
 		 $(call make-kernel-target,alldefconfig)
 
 ifeq (true,$(filter true, $(TARGET_NEEDS_DTBOIMAGE) $(BOARD_KERNEL_SEPARATED_DTBO)))
+ifneq ($(BOARD_CUSTOM_DTBOIMG_MK),)
+include $(BOARD_CUSTOM_DTBOIMG_MK)
+else
 MKDTIMG := $(HOST_OUT_EXECUTABLES)/mkdtimg$(HOST_EXECUTABLE_SUFFIX)
 MKDTBOIMG := $(HOST_OUT_EXECUTABLES)/mkdtboimg.py$(HOST_EXECUTABLE_SUFFIX)
 $(BOARD_PREBUILT_DTBOIMAGE): $(DTC) $(MKDTIMG) $(MKDTBOIMG)
@@ -316,6 +321,7 @@ $(BOARD_PREBUILT_DTBOIMAGE):
 	$(call make-dtbo-target,$(KERNEL_DEFCONFIG))
 	$(call make-dtbo-target,dtbo.img)
 endif # BOARD_KERNEL_SEPARATED_DTBO
+endif # BOARD_CUSTOM_DTBOIMG_MK
 endif # TARGET_NEEDS_DTBOIMAGE/BOARD_KERNEL_SEPARATED_DTBO
 
 ifeq ($(BOARD_INCLUDE_DTB_IN_BOOTIMG),true)