[cmdline] Accept "netX" in iPXE commands

Allow any iPXE command expecting a network device name to accept
"netX" as a synonym for "most recently opened network device".

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

src/net/netdevice.c

 struct net_device * find_netdev ( const char *name ) {
 	struct net_device *netdev;
 
+	/* Allow "netX" shortcut */
+	if ( strcmp ( name, "netX" ) == 0 )
+		return last_opened_netdev();
+
+	/* Identify network device by name */
 	list_for_each_entry ( netdev, &net_devices, list ) {
 		if ( strcmp ( netdev->name, name ) == 0 )
 			return netdev;