/usr/bin/mkbimage is in grub-legacy 0.97-72.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #!/bin/bash
set -e
cat << EOF
This program is broken, unsupported upstream, and has been deprecated in favour
of grub-mkrescue (grub-pc package).
EOF
exit 0
|