compile mkimage rather than use precompiled executable

thanks to ewpa:
https://github.com/ewpa/duo-buildroot-sdk/commit/b19ae91d982cf0358f9e8172b845f3704601c680

build/Makefile

@@ -420,7 +420,7 @@ ifeq ($(CONFIG_KERNEL_ENTRY_HACK),y)
 	${Q}sed -i "s/load = <0x0 0x.*>;/load = <0x0 $(CONFIG_KERNEL_ENTRY_HACK_ADDR)>;/g" ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its
 	${Q}sed -i "s/entry = <0x0 0x.*>;/entry = <0x0 $(CONFIG_KERNEL_ENTRY_HACK_ADDR)>;/g" ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its
 endif
-	$(COMMON_TOOLS_PATH)/prebuild/mkimage -f ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its -k $(RAMDISK_PATH)/keys -r ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/boot.itb
+	${UBOOT_PATH}/build/${CHIP}_${BOARD}/tools/mkimage -f ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its -k $(RAMDISK_PATH)/keys -r ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/boot.itb
 
 ramboot: kernel-dts
 	$(call print_target)
@@ -437,7 +437,7 @@ ifeq ($(CONFIG_KERNEL_ENTRY_HACK),y)
 	${Q}sed -i "s/load = <0x0 0x.*>;/load = <0x0 $(CONFIG_KERNEL_ENTRY_HACK_ADDR)>;/g" ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its
 	${Q}sed -i "s/entry = <0x0 0x.*>;/entry = <0x0 $(CONFIG_KERNEL_ENTRY_HACK_ADDR)>;/g" ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its
 endif
-	$(COMMON_TOOLS_PATH)/prebuild/mkimage -f ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its -k $(RAMDISK_PATH)/keys -r $(OUTPUT_DIR)/ramboot.itb
+	${UBOOT_PATH}/build/${CHIP}_${BOARD}/tools/mkimage -f ${RAMDISK_PATH}/${RAMDISK_OUTPUT_FOLDER}/multi.its -k $(RAMDISK_PATH)/keys -r $(OUTPUT_DIR)/ramboot.itb
 
 kernel-clean:
 	$(call print_target)