fff-network: Provide script with CPUPORT

This provides the CPUPORT which is needed for the gateway firmware.
The script is supposed to be sourced.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>