RC_Channel: add RC option to report CRSF LQ as RSSI

Co-author: Jules Gilson <julesgilson@yahoo.co.uk>

libraries/RC_Channel/RC_Channel.h

@@ -507,6 +507,10 @@ public:
         return _options & uint32_t(Option::MULTI_RECEIVER_SUPPORT);
     }
 
+    bool use_crsf_lq_as_rssi(void) const {
+        return get_singleton() != nullptr && (_options & uint32_t(Option::USE_CRSF_LQ_AS_RSSI)) != 0;
+    }
+
     // returns true if overrides should time out.  If true is returned
     // then returned_timeout_ms will contain the timeout in
     // milliseconds, with 0 meaning overrides are disabled.
@@ -570,6 +574,7 @@ protected:
         CRSF_CUSTOM_TELEMETRY   = (1U << 8), // use passthrough data for crsf telemetry
         SUPPRESS_CRSF_MESSAGE   = (1U << 9), // suppress CRSF mode/rate message for ELRS systems
         MULTI_RECEIVER_SUPPORT  = (1U << 10), // allow multiple receivers
+        USE_CRSF_LQ_AS_RSSI     = (1U << 11), // returns CRSF link quality as RSSI value, instead of RSSI
     };
 
     void new_override_received() {

libraries/RC_Channel/RC_Channels_VarInfo.h

@@ -89,7 +89,7 @@ const AP_Param::GroupInfo RC_Channels::var_info[] = {
     // @DisplayName: RC options
     // @Description: RC input options
     // @User: Advanced
-    // @Bitmask: 0:Ignore RC Receiver, 1:Ignore MAVLink Overrides, 2:Ignore Receiver Failsafe bit but allow other RC failsafes if setup, 3:FPort Pad, 4:Log RC input bytes, 5:Arming check throttle for 0 input, 6:Skip the arming check for neutral Roll/Pitch/Yaw sticks, 7:Allow Switch reverse, 8:Use passthrough for CRSF telemetry, 9:Suppress CRSF mode/rate message for ELRS systems,10:Enable multiple receiver support
+    // @Bitmask: 0:Ignore RC Receiver, 1:Ignore MAVLink Overrides, 2:Ignore Receiver Failsafe bit but allow other RC failsafes if setup, 3:FPort Pad, 4:Log RC input bytes, 5:Arming check throttle for 0 input, 6:Skip the arming check for neutral Roll/Pitch/Yaw sticks, 7:Allow Switch reverse, 8:Use passthrough for CRSF telemetry, 9:Suppress CRSF mode/rate message for ELRS systems,10:Enable multiple receiver support, 11:CRSF RSSI shows Link Quality
     AP_GROUPINFO("_OPTIONS", 33, RC_CHANNELS_SUBCLASS, _options, (uint32_t)RC_Channels::Option::ARMING_CHECK_THROTTLE),
 
     // @Param: _PROTOCOLS