netifd: add ubus event for link state

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 follow:

{ "network.device": {"ifname":"eth3","link":"down"} }

package/network/config/netifd/patches/500-add-device-ubus-event.patch

+diff --git a/device.c b/device.c
+index 1370335..279f2f7 100644
+--- a/device.c
++++ b/device.c
+@@ -956,6 +956,7 @@ void device_set_link(struct device *dev, bool state)
+ 	if (!state)
+ 		dev->auth_status = false;
+ 	device_broadcast_event(dev, state ? DEV_EVENT_LINK_UP : DEV_EVENT_LINK_DOWN);
++	netifd_ubus_device_event(dev, state);
+ }
+ 
+ void device_set_ifindex(struct device *dev, int ifindex)
+diff --git a/ubus.c b/ubus.c
+index e2aa3f8..d88a5ca 100644
+--- a/ubus.c
++++ b/ubus.c
+@@ -1402,6 +1402,15 @@ netifd_ubus_interface_event(struct interface *iface, bool up)
+ 	ubus_send_event(ubus_ctx, "network.interface", b.head);
+ }
+ 
++void
++netifd_ubus_device_event(struct device *dev, bool state)
++{
++	blob_buf_init(&b, 0);
++	blobmsg_add_string(&b, "ifname", dev->ifname);
++	blobmsg_add_string(&b, "link", state? "up" : "down");
++	ubus_send_event(ubus_ctx, "network.device", b.head);
++}
++
+ void
+ netifd_ubus_interface_notify(struct interface *iface, bool up)
+ {
+diff --git a/ubus.h b/ubus.h
+index dde7124..eccefed 100644
+--- a/ubus.h
++++ b/ubus.h
+@@ -27,4 +27,5 @@ void netifd_ubus_interface_event(struct interface *iface, bool up);
+ void netifd_ubus_interface_notify(struct interface *iface, bool up);
+ void netifd_ubus_device_notify(const char *event, struct blob_attr *data, int timeout);
+ 
++void netifd_ubus_device_event(struct device *dev, bool state);
+ #endif