[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/pnic.c

@@ -250,6 +250,9 @@ static int pnic_probe ( struct pci_device *pci,
 	status = pnic_command ( pnic, PNIC_CMD_READ_MAC, NULL, 0,
 				netdev->ll_addr, ETH_ALEN, NULL );
 
+	/* Mark as link up; PNIC has no concept of link state */
+	netdev_link_up ( netdev );
+
 	/* Register network device */
 	if ( ( rc = register_netdev ( netdev ) ) != 0 )
 		goto err;