meta-google: layer.conf: Cleanup for Yocto override syntax change

Update `_google-layer` to `:google-layer`.

Change-Id: I3443c265aa3ff3bb711f65c016942bfd53ecf153
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/meta-google/conf/layer.conf b/meta-google/conf/layer.conf
index ed605d1..86a965e 100644
--- a/meta-google/conf/layer.conf
+++ b/meta-google/conf/layer.conf
@@ -12,6 +12,6 @@
   "
 
 BBFILE_COLLECTIONS += "google-layer"
-BBFILE_PATTERN_google-layer := "^${LAYERDIR}/"
-LAYERVERSION_google-layer = "1"
-LAYERSERIES_COMPAT_google-layer = "gatesgarth hardknott"
+BBFILE_PATTERN:google-layer := "^${LAYERDIR}/"
+LAYERVERSION:google-layer = "1"
+LAYERSERIES_COMPAT:google-layer = "gatesgarth hardknott"