kernel: Define DTBS_OUT path for dtb.img building

Change-Id: I9fb3943c44c74985d9a4a841338767345a11ded2

build/tasks/kernel.mk

@@ -64,6 +64,7 @@ VARIANT_DEFCONFIG := $(TARGET_KERNEL_VARIANT_CONFIG)
 SELINUX_DEFCONFIG := $(TARGET_KERNEL_SELINUX_CONFIG)
 
 ## Internal variables
+DTBS_OUT := $(PRODUCT_OUT)/dtbs
 KERNEL_OUT := $(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ
 KERNEL_CONFIG := $(KERNEL_OUT)/.config
 KERNEL_RELEASE := $(KERNEL_OUT)/include/config/kernel.release
@@ -217,7 +218,7 @@ endef
 # Make a DTB targets
 # $(1): The DTB target to build (eg. dtbs, defconfig)
 define make-dtb-target
-$(call internal-make-kernel-target,$(PRODUCT_OUT)/dtbs,$(1))
+$(call internal-make-kernel-target,$(DTBS_OUT),$(1))
 endef
 
 $(KERNEL_OUT):