/etc/kernel/postinst.d/zz-u-boot-menu is in u-boot-menu 2.
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 8 9 10  | #!/bin/sh
set -e
# Exit if extlinux was removed (!= purged)
if [ -x /usr/sbin/u-boot-update ]
then
	# Update extlinux configuration
	u-boot-update
fi
 |