New build/util/gen-tarball for reproducibility (Closes: #805321)

Add a new utility build/util/gen-tarball to make reproducible
tarballs, and use it for:
- netboot.tar.gz
- debian-cd_info.tar.gz
- the overall debian-installer-images tar.gz file.

(cherry picked from commit c182491b)

Conflicts:
    debian/changelog

Signed-off-by: Cyril Brulebois <kibi@debian.org>