fff-network: Make custom port configuration update-safe

This writes CLIENT/BATMAN/WAN port configurations for devices
with switches to a separate file. This file can be changed
based on the desired setup and is then preserved during
firmware upgrade. In case of a future bugfix concerning the
entries of a specific network.* file, this will still be
effective as only the port configurations reside in the
new file, while the other settings could be overwritten by
the upgrade.

For one- and two-port devices, adjustments have been made so
that their relevant settings also reside in the new file now.

This also adds a WebUI interface for setting the two-port
devices (CPE210 and CPE510).

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Signed-off-by: Tim Niemeyer <tim@tn-x.org>
 - Changed doc in network.mode