ibm: Set LAYERSERIES_COMPAT

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

Change-Id: I592ecf6f9940004b2dccb0f4217464d389c771d4
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-romulus/conf/layer.conf b/meta-romulus/conf/layer.conf
index 18c57da..3a29f43 100644
--- a/meta-romulus/conf/layer.conf
+++ b/meta-romulus/conf/layer.conf
@@ -7,3 +7,4 @@
 
 BBFILE_COLLECTIONS += "romulus"
 BBFILE_PATTERN_romulus = ""
+LAYERSERIES_COMPAT_romulus = "sumo"