Add support for cross compiling Windows tools on Darwin.

Change-Id: I71372543e8da02e1d2437d6da7eb46b0a72c9870
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
cout: move to $out

Change-Id: I3b5bf8a5f62dcdf267662bb97a2073150152cfa6