atusb: updated and improved CAM script
atusb: updated and improved CAM script

- atusb/cam2/mkmk: updated for new board
- atusb/cam2/mkmk: further increased board to board spacing, to reduce
  board deflection
- atusb/cam2/mkmk: decreased nominal mill diameter to 20 mil to compensate
  for board/tool deflection
- atusb/cam2/mkmk: added target "dplot" to visualize the sequence in which
  holes are drilled

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