layer.conf: avoid unnecessary early expansion with :=

bitbake handles immediate expansions of LAYERDIR for us automatically.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
diff --git a/conf/layer.conf b/conf/layer.conf
index f208e42..ada915b 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -1,12 +1,12 @@
 # We have a conf and classes directory, add to BBPATH
-BBPATH := "${BBPATH}:${LAYERDIR}"
+BBPATH .= ":${LAYERDIR}"
 
 # We have a packages directory, add to BBFILES
-BBFILES := "${BBFILES} ${LAYERDIR}/recipes-*/*/*.bb \
+BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \
             ${LAYERDIR}/recipes-*/*/*.bbappend"
 
 BBFILE_COLLECTIONS += "telephony"
-BBFILE_PATTERN_telephony := "^${LAYERDIR}/"
+BBFILE_PATTERN_telephony = "^${LAYERDIR}/"
 BBFILE_PRIORITY_telephony = "5"
 
 META_TELEPHONY_OSMO_INC = "8"