Rover: sonar_trigger_cm of 0 will now only log

This will enable users of Rover to plug their sonar's in (or any other
device into the 3.3v ADC) and log the data but not use the data in
navigation and obstacle avoidance.

APMrover2/sensors.cpp

@@ -67,8 +67,8 @@ void Rover::read_sonars(void)
         // we have two sonars
         obstacle.sonar1_distance_cm = sonar.distance_cm(0);
         obstacle.sonar2_distance_cm = sonar.distance_cm(1);
-        if (obstacle.sonar1_distance_cm <= (uint16_t)g.sonar_trigger_cm &&
-            obstacle.sonar1_distance_cm <= (uint16_t)obstacle.sonar2_distance_cm)  {
+        if (obstacle.sonar1_distance_cm < (uint16_t)g.sonar_trigger_cm &&
+            obstacle.sonar1_distance_cm < (uint16_t)obstacle.sonar2_distance_cm)  {
             // we have an object on the left
             if (obstacle.detected_count < 127) {
                 obstacle.detected_count++;
@@ -79,7 +79,7 @@ void Rover::read_sonars(void)
             }
             obstacle.detected_time_ms = AP_HAL::millis();
             obstacle.turn_angle = g.sonar_turn_angle;
-        } else if (obstacle.sonar2_distance_cm <= (uint16_t)g.sonar_trigger_cm) {
+        } else if (obstacle.sonar2_distance_cm < (uint16_t)g.sonar_trigger_cm) {
             // we have an object on the right
             if (obstacle.detected_count < 127) {
                 obstacle.detected_count++;
@@ -95,7 +95,7 @@ void Rover::read_sonars(void)
         // we have a single sonar
         obstacle.sonar1_distance_cm = sonar.distance_cm(0);
         obstacle.sonar2_distance_cm = 0;
-        if (obstacle.sonar1_distance_cm <= (uint16_t)g.sonar_trigger_cm)  {
+        if (obstacle.sonar1_distance_cm < (uint16_t)g.sonar_trigger_cm)  {
             // obstacle detected in front 
             if (obstacle.detected_count < 127) {
                 obstacle.detected_count++;