conf/layer.conf: Use .= for BBPATH and += for BBFILES

Provide additional commentary that should help a bit more

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>