Do not hardcode /mnt/cdrom path across the whole code, use CDROOT_PATH

Do not hardcode /mnt/cdrom path across the whole code, use CDROOT_PATH
instead. At the same time, mount cdrom into /mnt/cdrom instead of
/newroot/mnt/cdrom (which is now just a bind mount), this avoids
losetup to expose unavailable paths inside the live system, breaking
mkfs.btrfs (next upstream version, which does silly things with
/proc/mounts).