[netdevice] Provide function to retrieve the most recently opened net device

There are currently four places within the codebase that use a
heuristic to guess the "boot network device", with varying degrees of
success.  Add a feature to the net device core to maintain a list of
open network devices, in order of opening, and provide a function
last_opened_netdev() to retrieve the most recently opened net device.
This should do a better job than the current assortment of
guess_boot_netdev() functions.

src/include/gpxe/netdevice.h

 	struct refcnt refcnt;
 	/** List of network devices */
 	struct list_head list;
+	/** List of open network devices */
+	struct list_head open_list;
 	/** Name of this network device */
 	char name[8];
 	/** Underlying hardware device */
 extern struct net_device * find_netdev ( const char *name );
 extern struct net_device * find_netdev_by_location ( unsigned int bus_type,
 						     unsigned int location );
+extern struct net_device * last_opened_netdev ( void );
 extern int net_tx ( struct io_buffer *iobuf, struct net_device *netdev,
 		    struct net_protocol *net_protocol, const void *ll_dest );
 extern int net_rx ( struct io_buffer *iobuf, struct net_device *netdev,

src/net/netdevice.c

 /** List of network devices */
 struct list_head net_devices = LIST_HEAD_INIT ( net_devices );
 
+/** List of open network devices, in reverse order of opening */
+struct list_head open_net_devices = LIST_HEAD_INIT ( open_net_devices );
+
 /**
  * Record network device statistic
  *
 
 	/* Mark as opened */
 	netdev->state |= NETDEV_OPEN;
+
+	/* Add to head of open devices list */
+	list_add ( &netdev->open_list, &open_net_devices );
+
 	return 0;
 }
 
 
 	/* Mark as closed */
 	netdev->state &= ~NETDEV_OPEN;
+
+	/* Remove from open devices list */
+	list_del ( &netdev->open_list );
 }
 
 /**
 	return NULL;	
 }
 
+/**
+ * Get most recently opened network device
+ *
+ * @ret netdev		Most recently opened network device, or NULL
+ */
+struct net_device * last_opened_netdev ( void ) {
+	struct net_device *netdev;
+
+	list_for_each_entry ( netdev, &open_net_devices, open_list ) {
+		assert ( netdev->state & NETDEV_OPEN );
+		return netdev;
+	}
+
+	return NULL;
+}
+
 /**
  * Transmit network-layer packet
  *