fff-sysupgrade: Add gateway config to sysupgrade.conf

User configuration for layer3 firmware is stored in
/etc/config/gateway. To preserve this file across updates it
is added to sysupgrade.conf

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