/etc/default/grub-imageboot is in grub-imageboot 0.6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | # Where to find the iso/floppy images
IMAGES="/boot/images"
# You can override the boot options for iso/floppy images here
# see http://syslinux.zytor.com/wiki/index.php/MEMDISK for details
#IMAGEOPTS="rawimg"
#ISOOPTS="iso"
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