Allow ADB to be enabled by default via a config overlay

This is useful for developers that are testing and troubleshooting boot issues.
This should only be used for testing and removed once a device is able to
boot.

Change-Id: Ib17d4e10ec68640ce55a91d5b1067d3d37eeec9d

Conflicts:
	core/res/res/values/config.xml