fs/iso9660: really create initrd temp dir

In case we're using an initrd, we create an empty "root" directory that
will contain only the bootloader stuff, not the actual root filesystem,
because it is in an initrd (standalone or initramfs).

We have to ensure that the directory is empty before assembling the
filesystem (to avoid any file lingering from a previous run, like the
sequence  "make; make"). So we first remove it before we create it, so
that on each build (especially not-from-scratch builds) we get the exact
expected content without any leftover.

However, the macro responsible for that, although defined since 7080eef9,
was never called.

Fix that by registering it as a pre-gen hook.

Note: the directory need not be created, as there are quite a few
"install -D" commands that ensure it is created. Yet, we prefer to
create it explicitly to avoid any confusion.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>