[build] don't print infos on stderr

generators should only output on stderr if there actually was an error.
Rather echo to stdout from the makefile to print the info.

This makes it possible to redirect stderr on compiling/testing to a buffer/file.
E.g. to check if there were warnings..

sw/tools/generators/gen_airframe.ml

@@ -319,7 +319,7 @@ let _ =
   and md5sum = Sys.argv.(3) in
   try
     let xml = start_and_begin xml_file h_name in
-    Xml2h.warning ("AIRFRAME MODEL: "^ ac_name);
+    (* Xml2h.warning ("AIRFRAME MODEL: "^ ac_name); *)
     define_string "AIRFRAME_NAME" ac_name;
     define "AC_ID" ac_id;
     define "MD5SUM" (sprintf "((uint8_t*)\"%s\")" (hex_to_bin md5sum));

sw/tools/generators/gen_flight_plan.ml

@@ -797,7 +797,7 @@ let () =
       end;
 
       let name = ExtXml.attrib xml "name" in
-      Xml2h.warning ("FLIGHT PLAN: "^name);
+      (* Xml2h.warning ("FLIGHT PLAN: "^name); *)
       Xml2h.define_string "FLIGHT_PLAN_NAME" name;
 
       let get_float = fun x -> float_attrib xml x in

sw/tools/generators/gen_radio.ml

@@ -145,7 +145,7 @@ let _ =
   nl ();
   let channels = Xml.children xml in
   let n = ExtXml.attrib xml "name" in
-  Xml2h.warning ("RADIO MODEL: "^n);
+  (* Xml2h.warning ("RADIO MODEL: "^n); *)
   define_string "RADIO_NAME" n;
   nl ();
   (*define "RADIO_CONTROL_NB_CHANNEL" (string_of_int (List.length channels));*)