layer.conf: Mark dunfell (3.1) release compatible

Signed-off-by: Khem Raj <raj.khem@gmail.com>

conf/layer.conf

@@ -9,6 +9,6 @@ BBFILE_COLLECTIONS += "swupdate"
 BBFILE_PATTERN_swupdate := "^${LAYERDIR}/"
 BBFILE_PRIORITY_swupdate = "6"
 
-LAYERSERIES_COMPAT_swupdate = "rocko sumo thud warrior zeus"
+LAYERSERIES_COMPAT_swupdate = "rocko sumo thud warrior zeus dunfell"
 
 LAYERDEPENDS_swupdate = "openembedded-layer"