AP_IOMCU: support RSSI from receiver

libraries/AP_IOMCU/AP_IOMCU.h

@@ -71,6 +71,11 @@ public:
     // get the name of the RC protocol
     const char *get_rc_protocol(void);
 
+    // get receiver RSSI
+    int16_t get_RSSI(void) const {
+        return rc_input.rssi;
+    }
+    
     /*
       get servo rail voltage
      */

libraries/AP_IOMCU/iofirmware/iofirmware.cpp

@@ -306,6 +306,7 @@ void AP_IOMCU_FW::rcin_update()
         }
         rc_last_input_ms = last_ms;
         rc_input.rc_protocol = (uint16_t)AP::RC().protocol_detected();
+        rc_input.rssi = AP::RC().get_RSSI();
     } else if (last_ms - rc_last_input_ms > 200U) {
         rc_input.flags_rc_ok = false;
     }

libraries/AP_IOMCU/iofirmware/ioprotocol.h

@@ -123,6 +123,7 @@ struct page_rc_input {
     uint8_t flags_rc_ok:1;
     uint8_t rc_protocol;
     uint16_t pwm[IOMCU_MAX_CHANNELS];
+    int16_t rssi;
 };
 
 /*