Modified LwIP stack and LwIP mbed-os wrapper to support sending IPv6

UDP packets when IPv4 and IPv6 dual mode stack is enabled and the IPv6
packet is sent over link-local inerface.

Author: Arun S

features/lwipstack/LWIPStack.cpp

@@ -439,7 +439,7 @@ nsapi_size_or_error_t LWIP::socket_sendto(nsapi_socket_t handle, const SocketAdd
     }
     if (netif_) {
         if ((addr.version == NSAPI_IPv4 && !get_ipv4_addr(netif_)) ||
-                (addr.version == NSAPI_IPv6 && !get_ipv6_addr(netif_))) {
+                (addr.version == NSAPI_IPv6 && !get_ipv6_addr(netif_) && !get_ipv6_link_local_addr(netif_))) {
             return NSAPI_ERROR_PARAMETER;
         }
     }

features/lwipstack/lwip_tools.cpp

@@ -273,11 +273,8 @@ bool convert_mbed_addr_to_lwip(ip_addr_t *out, const nsapi_addr_t *in)
 
 #if LWIP_IPV4 && LWIP_IPV6
     if (in->version == NSAPI_UNSPEC) {
-#if IP_VERSION_PREF == PREF_IPV4
-        ip_addr_set_zero_ip4(out);
-#else
-        ip_addr_set_zero_ip6(out);
-#endif
+        ip6_addr_set_zero(ip_2_ip6(out));
+        IP_SET_TYPE(out, IPADDR_TYPE_ANY);
         return true;
     }
 #endif