fs/iso9660: fix transparent (de)compression

fs/iso9660: fix transparent (de)compression

It needs mkzftree from zisofs-tools, so we add a dependency to it, and
we call that one explicitly (to avoid using the one from the host in
PATH).

It also needs the the uncompressed kernel image, but because it is
already in target/ so it gets compressed by mkzftree. We have two
options:
  - compress everything but the kernel image,
  - compress everything, kernel included, and recopy it later.

We choose the latter, because it is the simplest solution. So, we always
define the kernel-copy hook, but only register it when needed.

Finally, it needs a kernel with support for transparent
(de)compression, so we update the existing test config.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>