Bug#773881: live-boot: Fails to boot with union=overlayfs

[intrigeri <intrigeri@debian.org>]

Hi again,

>    The "workdir" needs to be an empty directory on the same filesystem
>    as upperdir.

In our case, upperdir is /live/overlay/, which itself is a tmpfs
mountpoint. I assume we can't put workdir *inside* upperdir, so to
satisfy the condition quoted above, we probably have to go on mounting
a tmpfs on /live/overlay/, but then use sub-directories within this
file hierarchy for upperdir and workdir, as opposed to using it
directly for upperdir.

I assume we can do that, as neither the /live/overlay/ path nor
/lib/live/mount/overlay apparently has never been part of the
documented live-boot's interface with the rest of the world, so it can
be considered to be an internal implementation detail.

If we go this way, then code needs to be adjusted accordingly at least
in the 2010-remove-persistence, 9990-misc-helpers.sh and
9990-overlay.sh components. Some stuff in live-tools might need
adjustments too.

Thoughts?

Cheers,
--
intrigeri