fs/iso9660: rename all variables to use the ROOTFS_ISO9660 prefix

For consistency reasons, this commit renames all internal variables of
iso9660.mk to use the ROOTFS_ISO9660 prefix.

While we're at it, replace a useless ':=' by '='.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>