build: Append -dtb to kernel image name if TARGET_KERNEL_APPEND_DTB is set

Currently devices that have TARGET_KERNEL_APPEND_DTB set also have to set
BOARD_KERNEL_IMAGE_NAME to Image.gz-dtb (or zImage-dtb on arm32) in order
to properly build the image with an appended DT. Automatially append -dtb
if TARGET_KERNEL_APPEND_DTB is set so BOARD_KERNEL_IMAGE_NAME does not need
to be set.

Change-Id: I5464c67092a7b9f86669902e5c0e4a47836f7cbd

build/tasks/kernel.mk

@@ -109,6 +109,9 @@ else
       TARGET_PREBUILT_INT_KERNEL_TYPE := zImage
     endif
   endif
+  ifeq ($(TARGET_KERNEL_APPEND_DTB),true)
+    TARGET_PREBUILT_INT_KERNEL_TYPE := $(TARGET_PREBUILT_INT_KERNEL_TYPE)-dtb
+  endif
 endif
 
 TARGET_PREBUILT_INT_KERNEL := $(KERNEL_OUT)/arch/$(KERNEL_ARCH)/boot/$(TARGET_PREBUILT_INT_KERNEL_TYPE)