fs/iso9660: change the name of the tmp dir to be consistent

Currently, the generated rootfs will be called rootfs.iso9660, but the
temporary directory we create while building it is called
rootfs-iso9660.tmp. They are in different directories so it's not so
obvious, but still to be consistent it's better to call the temp dir
rootfs.iso9660.tmp.

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