AP_Logger: log RC protocol each time a log is opened

libraries/AP_Logger/LoggerMessageWriter.cpp

@@ -181,6 +181,17 @@ void LoggerMessageWriter_WriteSysInfo::process() {
                 return; // call me again
             }
         }
+        stage = ws_blockwriter_stage_rc_protocol;
+        FALLTHROUGH;
+
+    case ws_blockwriter_stage_rc_protocol:
+        const char *prot = hal.rcin->protocol();
+        if (prot == nullptr) {
+            prot = "None";
+        }
+        if (! _logger_backend->Write_MessageF("RC Protocol: %s", prot)) {
+            return; // call me again
+        }
     }
 
     _finished = true;  // all done!

libraries/AP_Logger/LoggerMessageWriter.h

@@ -30,7 +30,8 @@ private:
         ws_blockwriter_stage_formats = 0,
         ws_blockwriter_stage_firmware_string,
         ws_blockwriter_stage_git_versions,
-        ws_blockwriter_stage_system_id
+        ws_blockwriter_stage_system_id,
+        ws_blockwriter_stage_rc_protocol
     };
     write_sysinfo_blockwriter_stage stage;
 };