build: Add support for extra kernel build variables

* This allows specifying a variant defconfig, and an selinux defconfig
   which are simply configuration fragments.

Merged from original commit:
https://github.com/CyanogenMod/android_build/commit/c102a65dfab65b7aed9d21377f238f0cb2cb78be
Merge remote-tracking branch 'upstream/jb-mr1' into jb-mr1