px4_log: Added documentation and handled unused variables

Added __attribute__ ((unused)) for variables used only for log
output and flagged as unused if the message log level is compiled out.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/commander/commander.cpp

@@ -361,7 +361,7 @@ int commander_main(int argc, char *argv[])
 
 	if (!strcmp(argv[1], "check")) {
 		int mavlink_fd_local = open(MAVLINK_LOG_DEVICE, 0);
-		int checkres = prearm_check(&status, mavlink_fd_local);
+		int checkres __attribute__ ((unused)) = prearm_check(&status, mavlink_fd_local);
 		close(mavlink_fd_local);
 		warnx("FINAL RESULT: %s", (checkres == 0) ? "OK" : "FAILED");
 		return 0;

src/modules/sdlog2/sdlog2.c

@@ -1936,8 +1936,8 @@ void sdlog2_status()
 	} else {
 
 		float kibibytes = log_bytes_written / 1024.0f;
-		float mebibytes = kibibytes / 1024.0f;
-		float seconds = ((float)(hrt_absolute_time() - start_time)) / 1000000.0f;
+		float mebibytes __attribute__ ((unused)) = kibibytes / 1024.0f;
+		float seconds __attribute__ ((unused)) = ((float)(hrt_absolute_time() - start_time)) / 1000000.0f;
 
 		warnx("wrote %lu msgs, %4.2f MiB (average %5.3f KiB/s), skipped %lu msgs", log_msgs_written, (double)mebibytes, (double)(kibibytes / seconds), log_msgs_skipped);
 		mavlink_log_info(mavlink_fd, "[sdlog2] wrote %lu msgs, skipped %lu msgs", log_msgs_written, log_msgs_skipped);

src/modules/sensors/sensors.cpp

@@ -706,7 +706,7 @@ Sensors::parameters_update()
 		warnx("WARNING     WARNING     WARNING\n\nRC CALIBRATION NOT SANE!\n\n");
 	}
 
-	const char *paramerr = "FAIL PARM LOAD";
+	const char *paramerr __attribute__ ((unused)) = "FAIL PARM LOAD";
 
 	/* channel mapping */
 	if (param_get(_parameter_handles.rc_map_roll, &(_parameters.rc_map_roll)) != OK) {