MT#4685 Try to fetch next device firmware

That way we sequentially upgrade the firmware from one to another,
because some firmware versions don't allow to be upgraded to the
latest one.

assets/linksys-spa5xx-configtemplate.tt

@@ -2,7 +2,7 @@
   [% barge_in = 0 -%]
   <Profile_Rule>[% config.url %]</Profile_Rule>
   <Upgrade_Enable>Yes</Upgrade_Enable>
-  <Upgrade_Rule>[% firmware.url %]</Upgrade_Rule>
+  <Upgrade_Rule>[% firmware.baseurl %]/$MA/from/$SWVER/next</Upgrade_Rule>
   <Station_Name>[% phone.stationname %]</Station_Name>
   <Station_Display_Name>[% phone.stationname %]</Station_Display_Name>
   <Server_Type>RFC3265_4235</Server_Type>