build: Clean up use of vendor/extra

 * extra_config.mk is not needed anymore
 * Include BoardConfigExtra.mk if available

Change-Id: Icd0db0852ceee5cb5c701c9a40b829e1f3ebd69e
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_build into android-4.4