atusb/fw/Makefile: introduce target-specific compliation variants
atusb/fw/Makefile: introduce target-specific compliation variants

- Makefile (USB_OBJS): moved into OBJS and BOOT_OBJS
- Makefile (OBJS): atu2.o is now app-atu2.o
- Makefile (BOOT_OBJS): atu2.o is now boot-atu2.o
- Makefile (MKDEP, %.o): moved dependency generation to macro $(MKDEP)
- Makefile (app-%.o): build from usb/%.c
- Makefile (boot -%.o): build from usb/%.c and set -DBOOT_LOADER

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