Correct LWIP::get_ip_address

Previous change that removed string-based APIs missed
`LWIP::get_ip_address`. Remove string-based method (which is not
overriding anything in `NetworkInterface`) and add missing binary form
to implement `NetworkInterface::get_ip_address`.

features/lwipstack/LWIPStack.cpp

@@ -191,30 +191,13 @@ LWIP::call_in_callback_cb_t LWIP::get_call_in_callback()
     return cb;
 }
 
-const char *LWIP::get_ip_address()
+nsapi_error_t LWIP::get_ip_address(SocketAddress *address)
 {
     if (!default_interface) {
-        return NULL;
+        return NSAPI_ERROR_NO_ADDRESS;
     }
 
-    const ip_addr_t *addr = get_ip_addr(true, &default_interface->netif);
-
-    if (!addr) {
-        return NULL;
-    }
-#if LWIP_IPV6
-    if (IP_IS_V6(addr)) {
-        return ip6addr_ntoa_r(ip_2_ip6(addr), ip_address, sizeof(ip_address));
-    }
-#endif
-#if LWIP_IPV4
-    if (IP_IS_V4(addr)) {
-        return ip4addr_ntoa_r(ip_2_ip4(addr), ip_address, sizeof(ip_address));
-    }
-#endif
-#if LWIP_IPV6 && LWIP_IPV4
-    return NULL;
-#endif
+    return get_ip_address_if(address, nullptr);
 }
 
 nsapi_error_t LWIP::get_ip_address_if(SocketAddress *address, const char *interface_name)

features/lwipstack/LWIPStack.h

@@ -93,7 +93,7 @@ public:
          */
         virtual char *get_mac_address(char *buf, nsapi_size_t buflen);
 
-        /** @copydoc NetworkStack::get_ip_address */
+        /** @copydoc OnboardNetworkStack::Interface::get_ip_address */
         virtual nsapi_error_t get_ip_address(SocketAddress *address);
 
         /** Get the IPv6 link local address in SocketAddress representation
@@ -304,19 +304,10 @@ public:
      */
     virtual nsapi_error_t add_dns_server(const SocketAddress &address, const char *interface_name);
 
-    /** Get the local IP address
-     *
-     *  @return         Null-terminated representation of the local IP address
-     *                  or null if not yet connected
-     */
-    virtual const char *get_ip_address();
+    /** @copydoc NetworkStack::get_ip_address */
+    virtual nsapi_error_t get_ip_address(SocketAddress *address);
 
-    /** Copies IP address of the name based network interface to user supplied buffer
-     *
-     * @param    address        SocketAddress object pointer to store the address
-     * @param    interface_name name of the interface
-     * @return                  Pointer to a buffer, or NULL if the buffer is too small
-     */
+    /** @copydoc NetworkStack::get_ip_address_if */
     virtual nsapi_error_t get_ip_address_if(SocketAddress *address, const char *interface_name);
 
     /** Set the network interface as default one