AP_HAL: add Link Quality reporting to RC protocols

libraries/AP_HAL/RCInput.h

@@ -34,7 +34,7 @@ public:
 
     /* get receiver based RSSI if available. -1 for unknown, 0 for no link, 255 for maximum link */
     virtual int16_t get_rssi(void) { return -1; }
-
+    virtual int16_t get_rx_link_quality(void) { return -1; }
     /* Return string describing method RC input protocol */
     virtual const char *protocol() const = 0;