device: add support for configuring vrf

device: add support for configuring vrf

Config example:
...
config device
	option type 'vrf'
	option name 'dcn'
	option table '20'
	option mtu '1500'
	option ipv6 '0'
	list ports 'l2tp-wanvpn'
	list ports 'lan3'

config interface 'dcn'
	option proto 'none'
	option device 'dcn'
...

Note: using "ports" in config simplifies luci integration

Tested-by: Paul Donald <newtwen+github@gmail.com>
Signed-off-by: Maxim Anisimov <maxim.anisimov.ua@gmail.com>
Link: https://github.com/openwrt/netifd/pull/38
Signed-off-by: Robert Marko <robimarko@gmail.com>