Plane: Make LAND_FLARE_SEC optional.

ArduPlane/Parameters.pde

@@ -235,7 +235,7 @@ const AP_Param::Info var_info[] PROGMEM = {
 
     // @Param: LAND_FLARE_SEC
     // @DisplayName: Landing flare time
-    // @Description: Vertical time before landing point at which to lock heading and flare with the motor stopped. This is vertical time, and is calculated based solely on the current height above the ground and the current descent rate.
+    // @Description: Vertical time before landing point at which to lock heading and flare with the motor stopped. This is vertical time, and is calculated based solely on the current height above the ground and the current descent rate.  Set to 0 if you only wish to flare based on altitude (see LAND_FLARE_ALT).
     // @Units: seconds
     // @Increment: 0.1
     // @User: Advanced

ArduPlane/landing.pde

@@ -45,15 +45,16 @@ static bool verify_land()
     /* Set land_complete (which starts the flare) under 3 conditions:
        1) we are within LAND_FLARE_ALT meters of the landing altitude
        2) we are within LAND_FLARE_SEC of the landing point vertically
-          by the calculated sink rate
+          by the calculated sink rate (if LAND_FLARE_SEC != 0)
        3) we have gone past the landing point and don't have
           rangefinder data (to prevent us keeping throttle on 
           after landing if we've had positive baro drift)
     */
     if (height <= g.land_flare_alt ||
-        height <= auto_state.land_sink_rate * aparm.land_flare_sec ||
-        (!rangefinder_state.in_range && location_passed_point(current_loc, prev_WP_loc, next_WP_loc)) ||
-        (fabsf(auto_state.land_sink_rate) < 0.2f && !is_flying())) {
+         (aparm.land_flare_sec != 0 && 
+          (height <= auto_state.land_sink_rate * aparm.land_flare_sec ||
+          (!rangefinder_state.in_range && location_passed_point(current_loc, prev_WP_loc, next_WP_loc)))) ||
+         (fabsf(auto_state.land_sink_rate) < 0.2f && !is_flying())  ) {
 
         if (!auto_state.land_complete) {
             if (!is_flying() && (hal.scheduler->millis()-auto_state.last_flying_ms) > 3000) {