Squashed commit of the following:

Networking:  Add implementation of logic for SIOCGIFCONF and SIOCGLIFCOF IOCTL commnds.
    Networking:  Add definitions and stuctures to support SIOCGIFCONF and SIOCGLIFCONF IOCTL commands.

net/netdev/netdev.h

@@ -344,6 +344,54 @@ void netdev_txnotify_dev(FAR struct net_driver_s *dev);
 int netdev_count(void);
 #endif
 
+/****************************************************************************
+ * Name: netdev_ipv4_ifconf
+ *
+ * Description:
+ *   Return the IPv4 configuration of each network adaptor
+ *
+ * Parameters:
+ *   ifc - A reference to the instance of struct ifconf in which to return
+ *         the information.
+ *
+ * Returned Value:
+ *   Zero is returned on success; a negated errno value is returned on any
+ *   failure.
+ *
+ * Assumptions:
+ *  The nework is locked
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_NET_IPv4
+struct ifconf;  /* Forward reference */
+int netdev_ipv4_ifconf(FAR struct ifconf *ifc);
+#endif
+
+/****************************************************************************
+ * Name: netdev_ipv6_ifconf
+ *
+ * Description:
+ *   Return the IPv6 configuration of each network adaptor
+ *
+ * Parameters:
+ *   lifc - A reference to the instance of struct lifconf in which to return
+ *          the information.
+ *
+ * Returned Value:
+ *   Zero is returned on success; a negated errno value is returned on any
+ *   failure.
+ *
+ * Assumptions:
+ *  The nework is locked
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_NET_IPv6
+struct lifconf;  /* Forward reference */
+int netdev_ipv6_ifconf(FAR struct lifconf *lifc);
+#endif
+
 /****************************************************************************
  * Name: netdev_dev_lladdrsize
  *