makefiles/Makefile.kicad: added target "overview"
makefiles/Makefile.kicad: added target "overview"

- Makefile.kicad (PHONY): added "overview"
- Makefile.kicad (overview): generate overview image and display it using
  ImageMagic's "display"

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