meta-phosphor: layer.conf: change BBFILE_PATTERN

Use the immmediate expansion operator when setting BBFILE_PATTERN.  This
makes meta-phosphor consistent with all the layers in meta-openembedded.

Change-Id: I9b23f494d40ed99e3a089843fbd1edfbcae7898c
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/layer.conf b/conf/layer.conf
index 6ffcde0..0df82a7 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -6,7 +6,7 @@
             ${LAYERDIR}/recipes-*/*/*.bbappend"
 
 BBFILE_COLLECTIONS += "phosphor-layer"
-BBFILE_PATTERN_phosphor-layer = "^${LAYERDIR}/"
+BBFILE_PATTERN_phosphor-layer := "^${LAYERDIR}/"
 LAYERVERSION_phosphor-layer = "1"
 LAYERSERIES_COMPAT_phosphor-layer = "thud warrior"