site.conf: opkg: Point OPKG package sources to http instead of https

Freifunk Nodes do not have https-support. So installing something via
opkg is not possible with the configuration provided with the images.
With this chage the package sources should now point to http.

OPKG uses signatures on the packages anyway, so we are fine with http
instead of https.

site.template

@@ -103,6 +103,9 @@ config_mode = {
 		show_altitude = true,
 	},
 },
+  opkg = {
+    openwrt = "http://downloads.openwrt.org/releases/%v/packages/%A",
+  },
 
 radv_filterd = {
   -- default setting. only here for reference..