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

Change how we search for CM makefiles.

build: Fix fastpath code for CM_BUILD

 * We need to adjust to the new validations

Change-Id: I9066d4f437beb9597027ee6bbb52504b5e7e84c5

build: Fix cm builds

the setup variable wasn't exported, and was being missed by part
of the scripts