[NETDEV] Add notion of link state

Add ability for network devices to flag link up/down state to the
networking core.

Autobooting code will now wait for link-up before attempting DHCP.

IPoIB reflects the Infiniband link state as the network device link state
(which is not strictly correct; we also need a succesful IPoIB IPv4
broadcast group join), but is probably more informative.

src/drivers/net/legacy.c

@@ -112,6 +112,9 @@ int legacy_probe ( void *hwdev,
 	 */
 	dev->desc.irq = nic.irqno;
 
+	/* Mark as link up; legacy devices don't handle link state */
+	netdev_link_up ( netdev );
+
 	if ( ( rc = register_netdev ( netdev ) ) != 0 )
 		goto err_register;