unity-greeter: fix wallpaper setup on release upgrade

helpers/DATA/unity-greeter/com.canonical.unity-greeter.gschema.xml

@@ -2,7 +2,7 @@
 <schemalist gettext-domain="unity-greeter">
   <schema id="com.canonical.unity-greeter" path="/com/canonical/unity-greeter/">
     <key name="background" type="s">
-      <default>'/usr/share/backgrounds/nabia.jpg'</default>
+      <default>'/usr/share/backgrounds/CODENAME'</default>
       <summary>Background file to use, either an image path or a color (e.g. #772953)</summary>
     </key>
     <key name="background-color" type="s">

helpers/make-unity-greeter

@@ -17,13 +17,16 @@
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 #
 
-VERSION=5
+VERSION=6
 
 . ./config
 
 # Custom settings
 cp $DATA/com.canonical.unity-greeter.gschema.xml data
 
+# Change the background wallpaper correspondingly to the new release along with extension.
+sed -i "s|CODENAME|$CODENAME.jpg|" data/com.canonical.unity-greeter.gschema.xml
+
 # Change the sound theme
 sed s/ubuntu/freedesktop/g src/unity-greeter.vala -i
 sed 's/"system-ready"/"bell"/' src/unity-greeter.vala -i