[lime] USB WAN feature proposal in LiMe

Hi all, before opening a PR on this subject I want to discuss this change with you, I implemented ubus APIs methods that can switch WAN interface from ethernet to usb and vice versa, giving the primary node of the mesh network WAN connectivity through usb tethering or 4G USB dongle, might be useful if there is no wired network where the routers are deployed or if the router model has only one wireless chip radio0 so the hotspot feature in LiMe cannot be used.
Let me know what you think and if it makes sense to open this PR.