fff-sysupgrade: Update sysupgrade.sh to support openwrt-18.06

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
[Set PKG_RELEASE back to 1]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>