Treat failure of ubus_add_object() in ubus_init() as retry-able.

3c93e8eb41 regularised ubus_init()
by avoiding logging calls (it can be called before logging is up)
but it instead returned any error from ubus_add_object() which
made such an error fatal. It turns out this is awkward, so this
patch returns NULL always, so that the event-loop will continue
attemping to connect to ubus forever.

This is not necessarily optimal either, and should be looked at
by a UBUS grown-up, but it does solve the immediate problem.

src/ubus.c

@@ -104,6 +104,8 @@ static void ubus_disconnect_cb(struct ubus_context *ubus)
     }
 }
 
+/* Note that this cannot log, it either returns a fatal error, or NULL.
+   If daemon->ubus is left as NULL, it will be called again for another try. */
 char *ubus_init()
 {
   struct ubus_context *ubus = NULL;
@@ -117,7 +119,7 @@ char *ubus_init()
   if (ret)
     {
       ubus_destroy(ubus);
-      return (char *)ubus_strerror(ret);
+      return NULL;
     }
       
   ubus->connection_lost = ubus_disconnect_cb;