HAL_Linux: support direct UDP output from UART drivers

this allows safe operation over WiFi links without MAVProxy

libraries/AP_HAL_Linux/UARTDriver.h

@@ -41,6 +41,7 @@ private:
     char *_ip;
     char *_flag;
     bool _connected; // true if a client has connected         
+    bool _packetise; // true if writes should try to be on mavlink boundaries
 
     // we use in-task ring buffers to reduce the system call cost
     // of ::read() and ::write() in the main loop
@@ -60,7 +61,16 @@ private:
     int _write_fd(const uint8_t *buf, uint16_t n);
     int _read_fd(uint8_t *buf, uint16_t n);
     void _tcp_start_connection(bool wait_for_connection);
-    int _parseDevicePath(char* arg);
+    void _udp_start_connection(void);
+
+    enum device_type {
+        DEVICE_TCP,
+        DEVICE_UDP,
+        DEVICE_SERIAL,
+        DEVICE_UNKNOWN
+    };
+
+    enum device_type _parseDevicePath(const char *arg);
     uint64_t _last_write_time;    
 };