tunnelmanager: babeld rx cost on core-routers too low

[Noki]

With the standard configuration of an uplink tunnel as described in our documentation we get the following RX cost on w38b-core:

The RX cost is only 96 which causes traffic to flow via the backup uplink tunnel of w38b-core towards zwingli-core, whenever saarbruecker-gw is used as tunnel server on w38b-core. The whole network currently looks as follows:

Expected behaviour would be to have a RX cost metric on the core router / TX cost metric on the gateway that is so high, that the tunnel would only be used if the wireless connection towards saarbruecker-gw breaks.

From my understanding this should be implemented as TX cost on the gateways. TX cost should probably be 1024 so TX cost and RX cost will be identical on the core routers for the wireguard tunnels.