config: add xfrm interface support scripts

config: add xfrm interface support scripts

This package adds scripts for xfrm interfaces support.
Example configuration via /etc/config/network:

config interface 'xfrm0'
        option proto 'xfrm'
        option mtu '1300'
        option zone 'VPN'
        option tunlink 'wan'
        option ifid 30

config interface 'xfrm0_static'
        option proto 'static'
        option ifname '@xfrm0'
        option ip6addr 'fe80::1/64'
        option ipaddr '10.0.0.1/30'

Now set in strongswan IPsec policy:
 	if_id_in = 30
	if_id_out = 30

Signed-off-by: André Valentin <avalentin@marcant.net>