IPStackInterface - add ip address

Currently only for IPv4. lwip was updated to accept 2 arguments for getting
the ip address (buffer and length).

features/net/FEATURE_IPV4/lwip-interface/LWIPIPStack.cpp

@@ -37,5 +37,5 @@ const char * IPStackInterface::get_mac_address()
 
 const char * IPStackInterface::get_ip_address()
 {
-    return lwip_get_ip_address();
+    return lwip_get_ip_address(_ip_address, sizeof(_ip_address));
 }

features/net/network-socket/IPStackInterface.h

@@ -52,6 +52,8 @@ public:
      * @return IP address in "10.11.12.13" form
      */
     virtual const char *get_ip_address();
+private:
+    char _ip_address[16]; //IPv4
 };
 
 #endif /* IP_STACK_INTERFACE_H */