device: add ubus event for link state change

[rthakur33]

Add a ubus event network.device for link state change to generate an event when the link goes up and down.

The event looks as follows: { "network.device": {"ifname":"eth3","link":"down"} }

[wryun]

This would be really useful, particularly since if you naively ubus subscribe to an object (the other way to get link info) you might lose the subscription if netifd decides to create a new object with the same name.