CPE210: Fix ROUTERMAC

The ROUTERMAC is not set in the network.* file. This causes a problem
when the ULA addresses are set based on the br-mesh MAC. If we just
add the ROUTERMAC, everything should be fine.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>