Copter: bug fix to take-off in Loiter and AltHold

Always set the target altitude after take-off to be current altitude +
20cm.  This resolves a bug in which the target altitude could end up
being a couple of meters higher than the current altitude if the user
entered Loiter / Alt Hold mode before inertial nav altitude estimate had
settled.

ArduCopter/Attitude.pde

@@ -979,9 +979,8 @@ static void
 set_throttle_takeoff()
 {
     // set alt target
-    if (controller_desired_alt < current_loc.alt) {
-        controller_desired_alt = current_loc.alt + ALT_HOLD_TAKEOFF_JUMP;
-    }
+    controller_desired_alt = current_loc.alt + ALT_HOLD_TAKEOFF_JUMP;
+
     // clear i term from acceleration controller
     if (g.pid_throttle_accel.get_integrator() < 0) {
         g.pid_throttle_accel.reset_I();