Bug#778849: Support restoring initrd on shutdown and pivoting into it

[intrigeri <intrigeri@debian.org>]

Hi,

Lukas Wunner:
> Dracut, which provides linux-initramfs-tool and is thus an alternative
> to initramfs-tools, supports restoring the initrd on shutdown and
> pivoting into it:

I might try to come up with a hackish PoC for Tails soon (rationale
for the curious: we will soon start relying on the kernel's memory
poisoning to erase most memory on shutdown; this can only work if the
read-write branch of our aufs filesystem is unmounted on shutdown, and
switching to dracut is a longer-term project, so our options so far
are either hacking this support into initramfs-tools, or using
a dracut-generated initrd for shutdown only).

FWIW Arch Linux' mkinitcpio also does:
https://git.archlinux.org/mkinitcpio.git/tree/shutdown

Details of the needed interface can be found in:

 * https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface/
 * https://www.freedesktop.org/wiki/Software/systemd/RootStorageDaemons/
 * systemd-shutdown(8)

Cheers,
-- 
intrigeri