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/conf/layer.conf b/conf/layer.conf
index cd84378..a5c9082 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -7,3 +7,4 @@
 
 BBFILE_COLLECTIONS += "ibm"
 BBFILE_PATTERN_ibm = ""
+LAYERSERIES_COMPAT_ibm = "sumo"