Fix aokp make file inclusion logic

This is a combination of 5 commits. Based on CM
Revert "Revert "Load only the current product config makefile.""

This reverts commit b6443c4e1f762af971d1d404645f8a08853a672b.
https://github.com/AOKP/build/commit/b6443c4e1f762af971d1d404645f8a08853a672b

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
https://github.com/CyanogenMod/android_build/commit/d56394fce050e5093207961efb99bb1ecf081fcc
Author: rmmc

Change-Id: I2a5e70dda973a6fcdbba0d5e26b35b99d3f1aea2

Change how we search for CM makefiles.
https://github.com/CyanogenMod/android_build/commit/034d63574adae82da0efb0414e30b156925b7258
Author: Koush

build: Fix fastpath code for CM_BUILD

 * We need to adjust to the new validations
https://github.com/CyanogenMod/android_build/commit/767bece8cd3db89c42bcfc8eeb69f78bc09f76cc
Author: Cyanogen

Change-Id: I9066d4f437beb9597027ee6bbb52504b5e7e84c5

Fix aokp make file inclusion logic

Adapt to AOKP and change to AOKP_TARGET so it doesn't interfere with AOKP_BUILD variable
Author: dhacker29

PS3: Rebase

Change-Id: I9302cf1d742cace05a36110a0f373c5b04c6086c