[generator] move the extraction of ground alt before ref0

by computing the ref0 earlier is it also required to get the ground alt
earlier, otherwise the generated alt in enu frame is wrong

sw/tools/generators/gen_flight_plan.ml

@@ -981,7 +981,6 @@ let print_flight_plan_h = fun xml ref0 xml_file out_file ->
         end
     with _ -> ()
   end;
-  ground_alt := get_float "ground_alt";
   let home_mode_height = try
     max (get_float "home_mode_height") !security_height
     with _ -> !security_height in
@@ -1181,6 +1180,7 @@ let generate = fun flight_plan ?(check=false) ?(dump=false) xml_file out_fp ->
     | _ -> failwith ("Error: unkown wp_frame \"" ^ frame ^ "\". Use \"utm\" or \"ltp\"")
     in
 
+  ground_alt := float_attrib xml "ground_alt";
   let ref0 = {
     frame;
     utm0 = utm_of WGS84 !fp_wgs84;