kernel: Set full path to DTC

* This makes it use the latest source built dtc instead of the prebuilt,
  which is too old for coral/4.14
* Would be best to have it use the in-kernel DTC though,
  as that should always work for that particular kernel.
* When OUT_DIR_COMMON_BASE isn't set, this path is relative (i.e. out/),
  so append $(KERNEL_BUILD_OUT_PREFIX) to account for that.

Change-Id: I6ffe72a2fb061ee81b385f31f4acf682876b27c8

build/tasks/kernel.mk

@@ -209,6 +209,8 @@ PATH_OVERRIDE += $(TOOLS_PATH_OVERRIDE)
 
 KERNEL_ADDITIONAL_CONFIG_OUT := $(KERNEL_OUT)/.additional_config
 
+KERNEL_MAKE_FLAGS += DTC=$(KERNEL_BUILD_OUT_PREFIX)$(DTC)
+
 # Internal implementation of make-kernel-target
 # $(1): output path (The value passed to O=)
 # $(2): target to build (eg. defconfig, modules, dtbo.img)