fs/iso9660: convert to the filesystem infrastructure

This commit converts the iso9660 logic to the common rootfs
infrastructure. What previously prevented it from being converted is
that the iso9660 logic needed to remove a temporary folder after the
image has been created.

However, since Buildroot typically keeps build artefacts around, this
commit changes the logic to keep this temporary folder around. Thanks
to this change, converting to the common rootfs infrastructure becomes
possible.

In addition, the temporary folder is renamed from $(BUILD_DIR)/iso9660
to the more descriptive $(BUILD_DIR)/rootfs-iso9660.tmp.

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