Add USB tethering support via ubus methods

[usg-ishimura]

This PR adds WAN connection in LiMe through USB tethering or 4G USB dongle (not tested), in this way primary node can get WAN connection even if there is no wired network where the routers are deployed or there is only one wireless chip per router radio0 so the hotspot feature cannot be used. The implemented ubus APIs can be called like so:

ubus call wan-service get_wan_status '{}'
ubus call wan-service-admin set_wan_interface '{"ifname":"eth1"}'
ubus call wan-service-admin set_wan_interface '{"ifname":"usb0"}'

This feature works with the following kernel modules included at build time: kmod-usb-net and kmod-usb-net-rndis