These changes allow you to create two independent config at the same
time or separately. It will be possible to add support for grub-pc in
the future.

The scripts were named the same for syslinux and grub. Because of this,
scripts for syslinux were overwritten by grub scripts.

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.