kernel: clean up this madness

* WARNING: this intentionally breaks the build on devices who
  are not setting BOARD_KERNEL_IMAGE_NAME and are still setting
  TARGET_USES_UNCOMPRESSED_KERNEL or TARGET_KERNEL_APPEND_DTB

* The logic here is insanity and if all of this can be done
  by simply setting BOARD_KERNEL_IMAGE_NAME, why have all this
  extra cruft?

* Use cases:
  - Set a kernel image name without "-dtb" on the end, you get a kernel.
    Nothing more.
  - Set a kernel image name with "-dtb" on the end, you get an kernel
    with an appended dt.
  - Set a kernel image name without "-dtb" on the end, and set
    BOARD_KERNEL_SEPARATED_DT. You get a kernel and a separated dt.img.
  - Set a kernel image name with "-dtb" on the end, and set
    BOARD_KERNEL_SEPARATED_DT. You're crazy.

Change-Id: I5c3fc6eb727255684278ff562c0ee3faa7124b6b