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.

(From meta-phosphor rev: 8640c4a464677f2e446a13e67686bf1be336fbef)

Change-Id: I23f7fbbcc637e6c6e0a11956a703c0439f53ad4a
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/conf/layer.conf b/meta-phosphor/conf/layer.conf
index 6ffcde0..0df82a7 100644
--- a/meta-phosphor/conf/layer.conf
+++ b/meta-phosphor/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"