Makefile: Add a 'distbin' target.

Makefile: Add a 'distbin' target.

This creates a sigrok-firmware-fx2lafw-bin-x.y.z.tar.gz tarball which
only contains pre-built *.fw files (and some docs), but no source code
of fx2lafw.

This tarball can be provided as a convenience download for users who do
not want to mess with building their own firmware files from source.