build: Add support for additional kernel config snippet

 * Add support for TARGET_KERNEL_ADDITIONAL_CONFIG which will append a
   config snippet from arch/$ARCH/configs/ to the main defconfig.
 * This can be used for various things, such as including DIAG support
   into debuggable builds.

Change-Id: Ifa48688a3f951dd8ecc9a13a27de3a476e7bf633