atben/Makefile: replace defective mlztx process with more robust cptx
atben/Makefile: replace defective mlztx process with more robust cptx

- Makefile (MLZTX, mlztx): fails to update fields that exist but have
  older content; removed
- Makefile (CPTX, cptx): use more robust but less elegant cptx process
  for now

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan