build: Add support for extra kernel build variables

 * This allows specifying a variant defconfig, and an selinux defconfig
   which are simply configuration fragments.

Change-Id: I97882ae3b8c2e16ff6a7dce8dd3a70d70f8aa866

Conflicts:
	core/tasks/kernel.mk
Merge "build: Add support for extra kernel build variables" into jb4.2