[netdevice] Add concept of a network device index

IPv6 link-local socket addresses require some way to specify a local
network device.  We cannot simply use a pointer to the network device,
since a struct sockaddr_in6 may be long-lived and has no way to hold a
reference to the network device.

Using a network device index allows a socket address to cleanly refer
to a network device without worrying about whether or not that device
continues to exist.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/netdevice.h

@@ -300,6 +300,8 @@ struct net_device {
 	struct list_head list;
 	/** List of open network devices */
 	struct list_head open_list;
+	/** Index of this network device */
+	unsigned int index;
 	/** Name of this network device */
 	char name[12];
 	/** Underlying hardware device */