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
Merge remote-tracking branch 'm/jb-mr1' into jb-mr1-bluez

Change-Id: I7d2a5f048ce20f58b691400a174c1ce1c5d69f34