This value is used to authenticate rescue rootfs image
by verifying the squashfs file's sha256sum before use
(propagator-20140419+).

Looks like this check might be useful for other stage2
images as well but let's get started with this one.

Thanks Maxim Suhanov <suhanov/group-ib.ru> for both
http://www.forensicswiki.org/wiki/Forensic_Live_CD_issues
and propagator patches.

It's not exactly obvious how install2_size, live_size or rescue_size
get defined since the variable names themselves get constructed;
help git grep these down.

There's much reason for reuse instead of duplication
among the different stage2-based subprofiles.

In particular, the rather monolithic driver cleanup script
of the ancient is better done in several clear pieces with
the final depmod run.

Scripts dropping apt/rpm databases will dump pkglist first.

A script purging /boot/* will honour live-install if present.

Minor inno^Wfixups all over the map too.

Implementation based on m-p-d::profiles/scripts.d/03-syslinux
(but heavily modified for m-p of course)