Fixed recvfrom to return connected IP address

In the current version of the firmware, the esp8266 handles UDP
in an odd way. Rather than packet based transactions, the esp8266
handles UDP like a connection based protocol.

To return the correct address, the esp8266 just saves the last
connection established by UDP. Additionally this patch added
the ability of a socket to change connection addresses.

Note: This firmware is still unable to recieve connections from
arbitrary hosts.

Author: geky

ESP8266Interface.cpp

@@ -131,6 +131,7 @@ struct esp8266_socket {
     int id;
     nsapi_protocol_t proto;
     bool connected;
+    SocketAddress addr;
 };
 
 int ESP8266Interface::socket_open(void **handle, nsapi_protocol_t proto)
@@ -234,20 +235,35 @@ int ESP8266Interface::socket_recv(void *handle, void *data, unsigned size)
 int ESP8266Interface::socket_sendto(void *handle, const SocketAddress &addr, const void *data, unsigned size)
 {
     struct esp8266_socket *socket = (struct esp8266_socket *)handle;
+
+    if (socket->connected && socket->addr != addr) {
+        _esp.setTimeout(ESP8266_MISC_TIMEOUT);
+        if (!_esp.close(socket->id)) {
+            return NSAPI_ERROR_DEVICE_ERROR;
+        }
+        socket->connected = false;
+    }
+
     if (!socket->connected) {
         int err = socket_connect(socket, addr);
         if (err < 0) {
             return err;
         }
+        socket->addr = addr;
     }
 
     return socket_send(socket, data, size);
 }
 
 int ESP8266Interface::socket_recvfrom(void *handle, SocketAddress *addr, void *data, unsigned size)
 {
-    struct esp8266_socket *socket = (struct esp8266_socket *)handle;    
-    return socket_recv(socket, data, size);
+    struct esp8266_socket *socket = (struct esp8266_socket *)handle;
+    int ret = socket_recv(socket, data, size);
+    if (ret >= 0 && addr) {
+        *addr = socket->addr;
+    }
+
+    return ret;
 }
 
 void ESP8266Interface::socket_attach(void *handle, void (*callback)(void *), void *data)

ESP8266Interface.h

@@ -270,6 +270,7 @@ class ESP8266Interface : public NetworkStack, public WiFiInterface
         void (*callback)(void *);
         void *data;
     } _cbs[ESP8266_SOCKET_COUNT];
+    SocketAddress _udp_remotes[ESP8266_SOCKET_COUNT];
 };
 
 #endif