build: Preliminary support for CM product builds

If building a cm_* product, skip crawling the filesystem for
AndroidProduct makefiles (and, of course, parsing their contents)
and aim directly for the device/cm/<product> device configuration

Change-Id: I2a5e70dda973a6fcdbba0d5e26b35b99d3f1aea2

Conflicts:
	envsetup.sh