[hacks] Improve the guess_boot_netdev() logic

This function is a major kludge, but can be made slightly more
accurate by ignoring net devices that aren't open.  Eventually it
needs to be removed entirely.

src/arch/i386/image/nbi.c

  * @ret netdev		Boot network device
  */
 static struct net_device * guess_boot_netdev ( void ) {
-	struct net_device *boot_netdev;
+	struct net_device *netdev;
 
 	/* Just use the first network device */
-	for_each_netdev ( boot_netdev ) {
-		return boot_netdev;
+	for_each_netdev ( netdev ) {
+		if ( netdev->state & NETDEV_OPEN )
+			return netdev;
 	}
 
 	return NULL;

src/usr/aoeboot.c

  * @ret netdev		Boot network device
  */
 static struct net_device * guess_boot_netdev ( void ) {
-	struct net_device *boot_netdev;
+	struct net_device *netdev;
 
 	/* Just use the first network device */
-	for_each_netdev ( boot_netdev ) {
-		return boot_netdev;
+	for_each_netdev ( netdev ) {
+		if ( netdev->state & NETDEV_OPEN )
+			return netdev;
 	}
 
 	return NULL;

src/usr/iscsiboot.c

  * @ret netdev		Boot network device
  */
 static struct net_device * guess_boot_netdev ( void ) {
-	struct net_device *boot_netdev;
+	struct net_device *netdev;
 
 	/* Just use the first network device */
-	for_each_netdev ( boot_netdev ) {
-		return boot_netdev;
+	for_each_netdev ( netdev ) {
+		if ( netdev->state & NETDEV_OPEN )
+			return netdev;
 	}
 
 	return NULL;