atben/Makefile: added targets for generating and reviewing Gerbers
atben/Makefile: added targets for generating and reviewing Gerbers

- Makefile (gerber): generate Gerbers for all layers
- Makefile (gerbv): show the Gerbers in a "natural" stacking

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