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