HAL_PX4: support RSSI from receiver protocols

libraries/AP_HAL_PX4/RCInput.cpp

@@ -118,6 +118,10 @@ void PX4RCInput::_timer_tick(void)
     if (orb_check(_rc_sub, &rc_updated) == 0 && rc_updated) {
         pthread_mutex_lock(&rcin_mutex);
         orb_copy(ORB_ID(input_rc), _rc_sub, &_rcin);
+        if (_rcin.rssi != 0 || _rssi != -1) {
+            // always zero means not supported
+            _rssi = _rcin.rssi;
+        }
         pthread_mutex_unlock(&rcin_mutex);
     }
     // note, we rely on the vehicle code checking new_input()

libraries/AP_HAL_PX4/RCInput.h

@@ -18,6 +18,11 @@ public:
     uint16_t read(uint8_t ch) override;
     uint8_t read(uint16_t* periods, uint8_t len) override;
 
+    int16_t get_rssi(void) override {
+        return _rssi;
+    }
+        
+    
     bool set_overrides(int16_t *overrides, uint8_t len) override;
     bool set_override(uint8_t channel, int16_t override) override;
     void clear_overrides() override;
@@ -35,4 +40,5 @@ private:
     bool _override_valid;
     perf_counter_t _perf_rcin;
     pthread_mutex_t rcin_mutex;
+    int16_t _rssi = -1;
 };