Put EXE header and main chunk load header into a single file --

it makes no sense to be able to include/exclude them separately.

cfg/atari-asm.cfg

@@ -3,7 +3,6 @@ FEATURES {
 }
 SYMBOLS {
     __EXEHDR__:          type = import;
-    __MAINCHNKHDR__:     type = import;
     __AUTOSTART__:       type = import;  # force inclusion of autostart "trailer"
     __STARTADDRESS__:    type = export, value = %S;
 }