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>

package/network/config/xfrm/files/xfrm.sh

@@ -0,0 +1,65 @@
+#!/bin/sh
+
+[ -n "$INCLUDE_ONLY" ] || {
+	. /lib/functions.sh
+	. /lib/functions/network.sh
+	. ../netifd-proto.sh
+	init_proto "$@"
+}
+
+proto_xfrm_setup() {
+	local cfg="$1"
+	local mode="xfrm"
+
+	local tunlink ifid mtu zone
+	json_get_vars tunlink ifid mtu zone
+
+	proto_init_update "$cfg" 1
+
+	proto_add_tunnel
+	json_add_string mode "$mode"
+	json_add_int mtu "${mtu:-1280}"
+
+	[ -z "$tunlink" ] && {
+		proto_notify_error "$cfg" NO_TUNLINK
+		proto_block_restart "$cfg"
+		exit
+	}
+	json_add_string link "$tunlink"
+
+	[ -z "$ifid" ] && {
+		proto_notify_error "$cfg" NO_IFID
+		proto_block_restart "$cfg"
+		exit
+	}
+	json_add_object 'data'
+	[ -n "$ifid" ] && json_add_int ifid "$ifid"
+	json_close_object
+
+	proto_close_tunnel
+
+	proto_add_data
+	[ -n "$zone" ] && json_add_string zone "$zone"
+	proto_close_data
+
+	proto_send_update "$cfg"
+}
+
+proto_xfrm_teardown() {
+	local cfg="$1"
+}
+
+proto_xfrm_init_config() {
+	no_device=1
+	available=1
+
+	proto_config_add_int "mtu"
+	proto_config_add_string "tunlink"
+	proto_config_add_string "zone"
+	proto_config_add_int "ifid"
+}
+
+
+[ -n "$INCLUDE_ONLY" ] || {
+	[ -f /lib/modules/$(uname -r)/xfrm_interface.ko -o -d /sys/module/xfrm_interface ] && add_protocol xfrm
+}