Add support for mediatek platforms

This includes optional support for building the kernel with mediatek's
build system, which is usually included with OEM source drops for this
platform. (enabled by BOARD_USES_MTK_KERNELBUILD:=true)

Change-Id: I69fb50aa17d9c171bf8a7c220a0707c4bc570733

kernel: remove support for unused MTK Kernel

Remove unused MTK kernel build support.

Change-Id: I4f4df078ae03b1bd5edc7000e5b942bfc05c02bd