- Makefile: removed limitation to Ben and added host build
- atusb.c: atusb-specific acquisition and calculations
- atrf-xtal.h (do_atusb): interface to atusb.c
- atrf-xtal.c (atben, atusb, main): added switch to board-specific driver
- atrf-xtal.c (usage, main): added support for atusb
