dmsquash-live: add support for "iso-scan/filename" kernel parameter

now you can write grub entries like

  set isofile="/Fedora-live.iso"
  loopback loop $isofile
  linux loop)/isolinux/vmlinuz iso-scan/filename=$isofile root=live:CDLABEL=Fedora-...
  initrd (loop)/isolinux/initrd0.img