openwrt_core:Added script to disable port forwarding
Adds a firewall script to be included in the tr181 implementation of Device.Firewall.Level.PortMappingEnabled to flush port forwarding rules if any.
Since 1-o-1 mapping of this parameter to something in uci does not exist we take this path.
Note: No change to default firewall config needed. The bbf implementation takes care of config.