[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/natsemi.c

@@ -205,6 +205,9 @@ static int natsemi_probe (struct pci_device *pci,
 		last = last1;
 	}
 
+	/* Mark as link up; we don't yet handle link state */
+	netdev_link_up ( netdev );
+
 	if ((rc = register_netdev (netdev)) != 0)
 		goto err_register_netdev;