/lib/live/config/9990-hooks is in open-infrastructure-system-config 20161101-lts1-2.
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 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84  | #!/bin/sh
## live-config(7) - System Configuration Components
## Copyright (C) 2006-2015 Daniel Baumann <mail@daniel-baumann.ch>
##
## This program comes with ABSOLUTELY NO WARRANTY; for details see COPYING.
## This is free software, and you are welcome to redistribute it
## under certain conditions; see COPYING for details.
#set -e
Cmdline ()
{
	# Reading kernel command line
	for _PARAMETER in ${LIVE_CONFIG_CMDLINE}
	do
		case "${_PARAMETER}" in
			live-config.hooks=*|hooks=*)
				LIVE_HOOKS="${_PARAMETER#*hooks=}"
				;;
		esac
	done
}
Init ()
{
	if [ -z "${LIVE_HOOKS}" ]
	then
		exit 0
	fi
	echo -n " hooks"
}
Config ()
{
	for _HOOK in $(echo ${LIVE_HOOKS} | sed -e 's/|/ /g')
	do
		case "${_HOOK}" in
			filesystem)
				if ls /lib/live/config-hooks/* 2>&1
				then
					_HOOKS="${_HOOKS} $(for _FILE in /lib/live/config-hooks/*; do echo file://${_FILE}; done)"
				fi
				;;
			medium)
				if ls /lib/live/mount/medium/live/config-hooks/* 2>&1
				then
					_HOOKS="${_HOOKS} $(for _FILE in /lib/live/mount/medium/live/config-hooks/*; do echo file://${_FILE}; done)"
				fi
				;;
			*)
				_HOOKS="${_HOOKS} ${_HOOK}"
				;;
		esac
	done
	for _HOOK in ${_HOOKS}
	do
		_TMPFILE="$(mktemp -t live-config.XXXXXXXX)"
		if echo "${_HOOK}" | grep -qs file://
		then
			# local file
			cp $(echo ${_HOOK} | sed 's|file://||') "${_TMPFILE}"
		else
			# remote file
			Setup_network
			wget --quiet "${_HOOK}" -O "${_TMPFILE}"
		fi
		chmod 0755 "${_TMPFILE}"
		"${_TMPFILE}"
		rm -f "${_TMPFILE}"
	done
}
Cmdline
Init
Config
 |