Build kernel with 4.7

Let's not cause issues with kernels that can't build above 4.7 right now
Remove hardcoded toolchains

Change-Id: Ib2892208423b6b9798f729e8370e3dd2c7ee408c
Be sure that cache getting wiped too

Change-Id: Idbf3a07243bde3e18f65a841b16390342c19b6bb

Conflicts:
	tools/releasetools/ota_from_target_files
Merge remote-tracking branch 'th/kitkat' into kitkat