atusb: Build updates

Update the root level Makefile to both invoke "make atusb.dfu" when
building.

Update the INSTALL document to mention the dependency on dfu-util.

Update the root level Makefile to install atusb.dfu if it's compiled.
Even though the kernel itself doesn't load the firmware into the
device this will ensure that pre-compiled firmware is included in the
release tarballs.