aspeed: Set LAYERSERIES_COMPAT

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

Change-Id: I99921e6d710135b3eed4d23363a2b924af4906f6
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/layer.conf b/conf/layer.conf
index 79933c2..613c41c 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -7,3 +7,4 @@
 BBFILE_COLLECTIONS += "aspeed-layer"
 BBFILE_PATTERN_aspeed-layer = ""
 LAYERVERSION_aspeed-layer = "1"
+LAYERSERIES_COMPAT_aspeed-layer = "sumo"