phosphor: Set LAYERSERIES_COMPAT

Starting with sumo, bitbake emits a warning if this variable is
not set.  Indicate sumo branch compatibility.

Change-Id: I0832841e92aea4e707131221b8992f8999da395a
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/layer.conf b/conf/layer.conf
index 2510f30..c905466 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -8,6 +8,7 @@
 BBFILE_COLLECTIONS += "obmc-phosphor"
 BBFILE_PATTERN_obmc-phosphor = "^${LAYERDIR}/"
 LAYERVERSION_obmc-phosphor = "1"
+LAYERSERIES_COMPAT_obmc-phosphor = "sumo"
 
 # Provide a variable that points the base of the phosphor layer.
 PHOSPHORBASE = '${@os.path.normpath("${LAYERDIR}/")}'