Rover: pivot steering stays active until within 10deg of target heading

Previously the vehicle would pop out of pivot-steering once it was within PIVOT_TURN_ANGLE degrees of the target heading.  This meant that it would give up on the pivot steer long before it was actually pointing at the target.

APMrover2/Rover.h

@@ -367,6 +367,9 @@ private:
     // set if the users asks for auto reverse
     bool in_auto_reverse;
 
+    // true if pivoting (set by use_pivot_steering)
+    bool pivot_steering_active;
+
     static const AP_Scheduler::Task scheduler_tasks[];
 
     // use this to prevent recursion during sensor init

APMrover2/Steering.cpp

@@ -62,14 +62,31 @@ bool Rover::auto_check_trigger(void) {
 /*
     work out if we are going to use pivot steering
 */
-bool Rover::use_pivot_steering(void) {
-    if (control_mode >= AUTO && have_skid_steering() && g.pivot_turn_angle != 0) {
-        const int16_t bearing_error = wrap_180_cd(nav_controller->target_bearing_cd() - ahrs.yaw_sensor) / 100;
-        if (abs(bearing_error) > g.pivot_turn_angle) {
-            return true;
-        }
+bool Rover::use_pivot_steering(void)
+{
+    // check cases where we clearly cannot use pivot steering
+    if (control_mode < AUTO || !have_skid_steering() || g.pivot_turn_angle <= 0) {
+        pivot_steering_active = false;
+        return false;
     }
-    return false;
+
+    // calc bearing error
+    const int16_t bearing_error = wrap_180_cd(nav_controller->target_bearing_cd() - ahrs.yaw_sensor) / 100;
+
+    // if error is larger than pivot_turn_angle start pivot steering
+    if (bearing_error > g.pivot_turn_angle) {
+        pivot_steering_active = true;
+        return true;
+    }
+
+    // if within 10 degrees of the target heading, exit pivot steering
+    if (bearing_error < 10) {
+        pivot_steering_active = false;
+        return false;
+    }
+
+    // by default stay in
+    return pivot_steering_active;
 }
 
 /*