Add ability to use variant defconfig for device differences

and use shared defconfig

Change-Id: Ie49e84a54e9758a661edfde6076ec0fc0929c047
defconfig: Update per http://source.android.com/devices/tech/kernel.html

Change-Id: I4ea1a4056f5c7824e2d0c41c5cf2834480edd7ad