strongswan: add interface uci list
The interface config option allows users to configure logical OpenWRT
interface names in the ipsec section; it allows StrongSwan to listen
and send traffic on specified interface(s). It translates to interfaces_use
StrongSwan option which is a comma sepearted list of network devices
that should be used by charon.
Since StrongSwan can only be started when one of the specified logical
OpenWRT interface is up procd interface triggers are installed to
trigger the reload script.
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>