prod/doc/Makefile: add missing dependencies on at{ben,usb}-{front,back}.png
prod/doc/Makefile: add missing dependencies on at{ben,usb}-{front,back}.png

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