build: Clean up use of vendor/extra

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

Change-Id: Icd0db0852ceee5cb5c701c9a40b829e1f3ebd69e
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0
Merge https://github.com/androidarmv6/android_build into cm-11.0