[Tails-dev] Smaller IUK packages

You can make the IUK packages smaller by replacing
rsync --archive --quiet --delete-after
with
rsync -rlpgoD --checksum --quiet --delete-after

This since many files currently is copied even if they actually haven't
changed, only had their timestamp updated (maybe because timestamp is set
to date the dpkg package was installed, because generated by a script, or
something). By the above modification I produced a squashfs delta between
Tails 0.18 and Tails 0.21 that was just 187 MB, something "xdelta" between
the ISO files confirmed is the best one can do without further tricks.

I don't know how large that squashfs delta was before the change, because
my computer OOM'ed every time I tried building it (tmpfs, and too little
ram).

I have *not* actually tried running the produced squashfs delta.
Timestamps is not updated anymore with this change, but I doubt any
application relies on them. But this has to be tested by whoever
investigate this further.