makefiles/Makefile.kicad: also show drill holes in PNG illustrations
makefiles/Makefile.kicad: also show drill holes in PNG illustrations

- Makefile.kicad (DRL2GERBER, %-Drill.gbr): convert Excellon drill file to
  Gerber
- Makefile.kicad ($(NAME)-front.png, $(NAME)-back.png,
  $(NAME)-overview.png): use the converted drill file to show holes
- Makefile.kicad (clean): remove $(NAME)-Drill.gbr

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