This merge request adds support for the Debian Installer to build reproducible
[0] installation media if a suitable SOURCE_DATE_EPOCH[1] variable is exported
to the environment.

My own testing — albeit limited to x86 architectures — indicates that all image
types (netboot, ISO, hd-media etc.) become bit-for-bit reproducible with this
changeset.

Note that it currently requires a `mtools` that has my patches from #900409 [2]
and #900410 [3] applied (available since 4.0.18-2.1).


 [0] https://reproducible-builds.org/
 [1] https://reproducible-builds.org/specs/source-date-epoch/
 [2] https://bugs.debian.org/900409
 [3] https://bugs.debian.org/900410

Extra review/discussion happening on https://bugs.debian.org/900918#34 (etc.)