Demo: the edited grub.cfg
Commands or hints:
unzip clonezilla-live-3.1.3-16-amd64.zip -d zip
cd zip
edit boot/grub/grub.cfg (e.g., vim boot/grub/grub.cfg)
xorriso -as mkisofs -R -r -J -joliet-long -l -cache-inodes -iso-level 3 -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin -partition_offset 16 -A 'Clonezilla live CD' -b syslinux/isolinux.bin -c syslinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -eltorito-alt-boot --efi-boot boot/grub/efi.img -isohybrid-gpt-basdat -isohybrid-apm-hfsplus ./ > /tmp/clonezilla-live.iso