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

Fixes parsing errors which is appearing after this commit to
meta-openembedded

http://cgit.openembedded.org/meta-openembedded/commit/?id=3c21a46020bd0816579648f684c41dbd6333583e



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

Signed-off-by: Andrei Gherzan <andrei.gherzan@windriver.com>