AP_TECS: Allow lower min. speed on landing final

Previously the TECS never allowed for a speed demand outside of the
navigation airspeed range (AIRSPEED_MIN to AIRSPEED_MAX), even on
landing final. This can be too fast for touchdown if a conservative
AIRSPEED_MIN is used, which might be preferable for other phases of
flight.

This commit lowers the min. airspeed to AIRSPEED_STALL, if set, during
landing final only. Using a value of TECS_LAND_ARSPD or LAND_PF_ARSPD
lower than AIRSPEED_MIN, it enables slower touchdowns while ensuring the
rest of the flight remains safely at or above AIRSPEED_MIN.

Author: rubenp02

libraries/AP_TECS/AP_TECS.cpp

@@ -411,6 +411,10 @@ void AP_TECS::_update_speed(float DT)
     _TASmax   = MIN(_TASmax, aparm.airspeed_max * EAS2TAS);
     _TASmin   = aparm.airspeed_min * EAS2TAS;
 
+    if (_landing.is_on_final() && is_positive(aparm.airspeed_stall)) {
+        _TASmin = aparm.airspeed_stall * EAS2TAS;
+    }
+
     if (aparm.stall_prevention) {
         // when stall prevention is active we raise the minimum
         // airspeed based on aerodynamic load factor