network: add virtual tunnel interface (VTI) support

network: add virtual tunnel interface (VTI) support

This adds support for configuring VTI interfaces within /etc/config/network.
VTI interfaces are used to create IPsec tunnel interfaces. These interfaces
may be used for routing and other purposes.

Example config:
config interface 'vti1'
	option proto 'vti'
	option mtu '1500'
	option tunlink 'wan'
	option peeraddr '192.168.5.16'
	option zone 'VPN'
	option ikey 2
	option okey 2

config interface 'vti1_static'
	option proto 'static'
	option ifname '@vti1'
	option ipaddr '192.168.7.2/24'

The options ikey and okey correspond to the fwmark value of a ipsec policy.
The may be null if you do not want fwmarks.
Also peeraddr may be 0.0.0 if you want all ESP packets go through the
interface.
Example strongswan config:
conn vti
	left=%any
	leftcert=peer2.test.der
	leftid=@peer2.test
	right=192.168.5.16
	rightid=@peer3.test
	leftsubnet=0.0.0.0/0
	rightsubnet=0.0.0.0/0
	mark=2
	auto=route

Signed-off-by: André Valentin <avalentin@marcant.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48274 3c298f89-4303-0410-b956-a3cf2f4a3e73