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-openbmc-machines/meta-openpower/meta-ibm/meta-witherspoon/conf/layer.conf b/meta-openbmc-machines/meta-openpower/meta-ibm/meta-witherspoon/conf/layer.conf
index 01848da..6ea5041 100644
--- a/meta-openbmc-machines/meta-openpower/meta-ibm/meta-witherspoon/conf/layer.conf
+++ b/meta-openbmc-machines/meta-openpower/meta-ibm/meta-witherspoon/conf/layer.conf
@@ -7,3 +7,4 @@
 
 BBFILE_COLLECTIONS += "witherspoon"
 BBFILE_PATTERN_witherspoon = ""
+LAYERSERIES_COMPAT_witherspoon = "sumo"