layer.conf: avoid unnecessary early expansion with :=

bitbake handles immediate expansions of LAYERDIR for us automatically.

(From OE-Core rev: 643ac87145eba5dc105377041bc3deb163b62926)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>