tested on Ubuntu artful and Slackware 14.2

Preliminary Note

Make sure the needed partition has the bootable flag (this is surely required for the mbr.bin bootcode, not sure about gtpmbr.bin.

fdisk -l /dev/sda

Also assuming you do NOT have a separate partition for /boot. I have / only as ext4, which EXTLINUX is able to read. Very recent versions of EXTLINUX are also capable to read XFS partitions -- see the Building latest EXTLINUX section.

If you have XFS instead of EXT4 on Slackware Linux, see the From Scratch section below, as XFS support is only available since v5.01. Ubuntu has v6.03 but slackware has v4.07.

Install // Packages

Ubuntu artful,

apt install syslinux-utils extlinux

Avoiding Slackware 14.2's obsolete v4.07,

#slackpkg install syslinux

Install // From Scratch

Fetch the latest stable or testing SYSLINUX,

#git clone --recursive git://repo.or.cz/syslinux.git
#https://www.syslinux.org/wiki/index.php?title=Development
wget https://cdn.kernel.org/pub/linux/utils/boot/syslinux/syslinux-6.03.tar.gz
tar xzf syslinux-6.03.tar.gz 
cd syslinux-6.03

which nasm
ls -lhF /usr/include/uuid/uuid.h
time make -j4
#1m21s on workstation
make install

which extlinux
extlinux -v

Write the bootcode

fdisk -l /dev/sda | grep ^Disklabel

In case you are using mbr.bin, you will have to MAKE THE PARTITION BOOTABLE,

cfdisk /dev/sda

Ubuntu GPT,

dd if=/usr/lib/SYSLINUX/gptmbr.bin of=/dev/sda

Ubuntu MBR,

dd if=/usr/lib/SYSLINUX/mbr.bin of=/dev/sda

Debian GPT,

dd if=/usr/lib/EXTLINUX/gptmbr.bin of=/dev/sda

From source GPT,

dd if=/usr/share/syslinux/gptmbr.bin of=/dev/sda

From source MBR,

dd if=/usr/share/syslinux/mbr.bin of=/dev/sda
#cat /usr/share/syslinux/mbr.bin > /dev/sda

Configure EXTLINUX

extlinux -i /

when the mount command does not show the tru device patch, force it e.g.,

#extlinux -i / --device /dev/sda1

ls -lhF /ldlinux.c32
ls -lhF /ldlinux.sys

#with multiboot/xen
cp -f /usr/share/syslinux/libcom32.c32 /
cp -f /usr/share/syslinux/mboot.c32 /
#debian
#cp /usr/lib/syslinux/modules/bios/libcom32.c32 /
#cp /usr/lib/syslinux/modules/bios/mboot.c32 /

#with menu
cp -f /usr/share/syslinux/libutil.c32 /
cp -f /usr/share/syslinux/menu.c32 /
#debian
#cp /usr/lib/syslinux/modules/bios/libutil.c32 /
#cp /usr/lib/syslinux/modules/bios/menu.c32 /

ls -altrhF /

vi /extlinux.conf

default XEN
prompt 1
timeout 30

#default menu.c32
#prompt 0
#menu title EXTLINUX

label Devuan
    #menu label ...
    linux vmlinuz
    append initrd=initrd.img root=/dev/sda1 ro

label XEN
    kernel mboot.c32
    append xen.gz dom0_mem=12288M pv-linear-pt=true --- vmlinuz console=tty0 root=/dev/sda1 ro --- initrd.img

#dom0_mem=4096M,max:8192M
#hap_1gb=false hap_2mb=false altp2m=1 flask_enforcing=1
#netcfg/do_not_use_netplan=true

Ready to check

sync
shutdown -r now

Clean-up

On Ubuntu artful,

apt purge lilo
dpkg -l | grep grub
apt purge ...grub...

On Slackware 14.2,

#no need to remove lilo, nothing is automated anyways
#and there is no grub

Improved Layout

#ubuntu
cp /slackware/usr/share/syslinux/menu.c32 /

ui menu.c32
prompt 0
menu title SYS
timeout 100
default XEN

References