openpower: Set LAYERSERIES_COMPAT

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

Change-Id: Ic95fb7929b2115c4659abf5632c4954cdd9a4440
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/layer.conf b/conf/layer.conf
index 6824543..7b01b6c 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -8,3 +8,4 @@
 BBFILE_COLLECTIONS += "openpower"
 BBFILE_PATTERN_openpower = "^${LAYERDIR}/"
 LAYERVERSION_openpower = "1"
+LAYERSERIES_COMPAT_openpower = "sumo"