build: allow multiple cm-device.mk in same repo

This is useful for repository consolidation when device unification is
not desired. roomservice references a device-to-repository json file to
check for known consolodations. This file syncs with the build repo to
ensure it is kept up-to-date and an internet connection is not required
just to build.

Two examples:

m7, m7vzw, and m7spr all represented by one repository:
https://github.com/mdmower/android_device_htc_m7/tree/cm-11.0-consol

ville and villedm (/data/media variant of ville):
https://github.com/mdmower/android_device_htc_ville/tree/cm-11.0-consol

Change-Id: I5c5a6d0819533dc69c85c6fdbc02200f6538122c