/lib/live/config/1140-xserver-xorg is in live-config 5.20170112+deb9u1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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## 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 ()
{
	for _PARAMETER in ${LIVE_CONFIG_CMDLINE}
	do
		case "${_PARAMETER}" in
			live-config.keyboard-layouts=*|keyboard-layouts=*)
				LIVE_KEYBOARD_LAYOUTS="${_PARAMETER#*keyboard-layouts=}"
				;;
			live-config.keyboard-model=*|keyboard-model=*)
				LIVE_KEYBOARD_MODEL="${_PARAMETER#*keyboard-model=}"
				;;
			live-config.keyboard-options=*|keyboard-options=*)
				LIVE_KEYBOARD_OPTIONS="${_PARAMETER#*keyboard-options=}"
				;;
			live-config.keyboard-variants=*|keyboard-variants=*)
				LIVE_KEYBOARD_VARIANTS="${_PARAMETER#*keyboard-variants=}"
				;;
			live-config.xorg-xsession-manager=*|x-session-manager=*)
				LIVE_X_SESSION_MANAGER="${_PARAMETER#*x-session-manager=}"
				;;
			live-config.xorg-driver=*|xorg-driver=*)
				LIVE_XORG_DRIVER="${_PARAMETER#*xorg-driver=}"
				;;
			live-config.xorg-resolution=*|xorg-resolution=*)
				LIVE_XORG_RESOLUTION="${_PARAMETER#*xorg-resolution=}"
				;;
		esac
	done
}
Init ()
{
	# Checking if package is installed
	if [ ! -e /var/lib/dpkg/info/xserver-xorg.list ] || \
	   [ -e /var/lib/live/config/xserver-xorg ]
	then
		exit 0
	fi
	echo -n " xserver-xorg"
}
Config ()
{
	if [ -n "${LIVE_KEYBOARD_MODEL}" ]
	then
		echo "xserver-xorg xserver-xorg/config/inputdevice/keyboard/model select ${LIVE_KEYBOARD_MODEL}" >> /tmp/live-config.cfg
	fi
	if [ -n "${LIVE_KEYBOARD_LAYOUTS}" ]
	then
		echo "xserver-xorg xserver-xorg/config/inputdevice/keyboard/layout select ${LIVE_KEYBOARD_LAYOUTS}" >> /tmp/live-config.cfg
	fi
	if [ -n "${LIVE_KEYBOARD_VARIANTS}" ]
	then
		echo "xserver-xorg xserver-xorg/config/inputdevice/keyboard/variant select ${LIVE_KEYBOARD_VARIANTS}" >> /tmp/live-config.cfg
	fi
	if [ -n "${LIVE_KEYBOARD_OPTIONS}" ]
	then
		echo "xserver-xorg xserver-xorg/config/inputdevice/keyboard/options string ${LIVE_KEYBOARD_OPTIONS}" >> /tmp/live-config.cfg
	fi
	if [ -n "${LIVE_X_SESSION_MANAGER}" ]
	then
		case "${LIVE_X_SESSION_MANAGER}" in
			none)
				_DM_FILE="/etc/X11/default-display-manager"
				if [ -e "${_DM_FILE}" ]
				then
					sed -i "s|\(.*\)|\1 # disabled by live-config:|" -- "${_DM_FILE}"
				fi
				;;
			*)
				update-alternatives --quiet --set x-session-manager "${LIVE_X_SESSION_MANAGER}"
				;;
		esac
	fi
	if [ "${LIVE_XORG_DRIVER}" = "noproprietary" ] ; then
		NOPROPRIETARY=true
		unset LIVE_XORG_DRIVER
	fi
	if [ -z "${LIVE_XORG_DRIVER}" ] && [ -e /usr/bin/lspci ]
	then
		# pci-id of the first graphic card
		_DEVICE="$(lspci -mn | awk '/0300/ { print $3$4 }' | sed -e 's|"||g' | tr [a-z] [A-Z] | head -n1)"
		if [ -n "${_DEVICE}" ]
		then
			if ls /usr/share/live/config/xserver-xorg/*.ids > /dev/null 2>&1
			then
				# xorg-driver manual overrides
				for _OVERRIDE_IDS in /usr/share/live/config/xserver-xorg/*.ids
				do
					if [ -e "${_OVERRIDE_IDS}" ]
					then
						if grep -qs "${_DEVICE}" "${_OVERRIDE_IDS}"
						then
							LIVE_XORG_DRIVER="$(basename ${_OVERRIDE_IDS} .ids)"
							break
						fi
					fi
				done
			fi
			if [ -z "${LIVE_XORG_DRIVER}" ] && [ -z "${NOPROPRIETARY}" ]
			then
				# xorg-driver automatic override for fglrx
				if echo "${_DEVICE}" | grep -qs '^1002'
				then
					if grep -qs "${_DEVICE}" /usr/share/fglrx/fglrx.ids
					then
						LIVE_XORG_DRIVER="fglrx"
					fi
				fi
				# xorg-driver automatic override for nvidia
				if echo "${_DEVICE}" | grep -qs -E '^(10DE|12D2)'
				then
					for _NVIDIA_IDS in /usr/lib/nvidia/current/nvidia.ids $(ls /usr/lib/nvidia/legacy-*/nvidia.ids | sort -V -r)
					do
						if [ -e "${_NVIDIA_IDS}" ]
						then
							if grep -qs "${_DEVICE}" ${_NVIDIA_IDS}
							then
								_NVIDIA_VERSION="$(basename $(dirname ${_NVIDIA_IDS}))"
								_NVIDIA_MODULE="$(echo nvidia-${_NVIDIA_VERSION} | sed -e 's|-current$||')"
								break
							fi
						fi
					done
					if [ -n "${_NVIDIA_VERSION}" ]
					then
						LIVE_XORG_DRIVER="nvidia"
					fi
				fi
			fi
		fi
	fi
	if [ -n "${LIVE_XORG_DRIVER}" ]
	then
		mkdir -p /etc/X11/xorg.conf.d
		if [ -e "/usr/share/live/config/xserver-xorg/${LIVE_XORG_DRIVER}.conf" ]
		then
			# xorg-driver manual override
			cp "/usr/share/live/config/xserver-xorg/${LIVE_XORG_DRIVER}.conf" /etc/X11/xorg.conf.d/zz-live-config_xserver-xorg.conf
		else
			# xorg-driver automatic override
cat > /etc/X11/xorg.conf.d/zz-live-config_xserver-xorg.conf << EOF
Section "Device"
	Identifier	"Default screen"
	Driver		"${LIVE_XORG_DRIVER}"
EndSection
EOF
		fi
		case "${LIVE_XORG_DRIVER}" in
			fglrx)
				update-alternatives --quiet --set glx /usr/lib/fglrx
				modprobe --ignore-install fglrx || true
				;;
			nvidia)
				update-alternatives --quiet --set glx /usr/lib/nvidia
				update-alternatives --quiet --set nvidia /usr/lib/nvidia/${_NVIDIA_VERSION:-current}
				modprobe --ignore-install ${_NVIDIA_MODULE:-nvidia} || true
				;;
			*)
				update-alternatives --quiet --set glx /usr/lib/mesa-diverted
				;;
		esac
	fi
	if [ -n "${LIVE_XORG_RESOLUTION}" ]
	then
		echo "xrandr -s ${LIVE_XORG_RESOLUTION} || /bin/true" >> /etc/X11/Xsession.d/21xvidemode
	else
		rm -f /etc/X11/Xsession.d/21xvidemode
	fi
	if [ -e /tmp/live-config.cfg ]
	then
		debconf-set-selections < /tmp/live-config.cfg
		rm -f /tmp/live-config.cfg
		dpkg-reconfigure -f noninteractive -p critical \
			xserver-xorg 2>&1 \
			| grep -v "overwriting possibly-customised configuration" \
			| grep -v "file; backup in /etc/X11/xorg.conf" || true
		# Creating state file
		touch /var/lib/live/config/xserver-xorg
	fi
}
Cmdline
Init
Config
 |