layer.conf: Use .= for adding to BBPATH and += to BBFILES

http://cgit.openembedded.org/meta-openembedded/commit/?id=3c21a46020bd0816579648

This triggers exception NameError: name 'base_contains' is not defined
without this change

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>