Merge pull request #1 from Jackeagle/kitkat

AOGP-fication
build: Clean up use of vendor/extra

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

Change-Id: Icd0db0852ceee5cb5c701c9a40b829e1f3ebd69e
Merge pull request #1 from AOGP/kitkat