Check for latest GRML ISO image
https://grml.org/download/

GRML_ISO=https://download.grml.org/grml64-small_2020.06.iso

# Downlaod Grml image
wget ${GRML_ISO} -qO /tmp/${GRML_ISO##*/}

# Mount ISO
mount /tmp/${GRML_ISO##*/} /mnt/ -o loop

# Copy GRML files to tftpboot directory
mkdir -p /var/lib/tftpboot/live/grml/64-small
cp /mnt/boot/grml64small/{initrd.img,vmlinuz} /var/lib/tftpboot/live/grml/64-small/
cp /mnt/live/grml64-small/grml64-small.squashfs /var/lib/tftpboot/live/grml/64-small/

# Umount ISO
umount /mnt

# Create iPXE config
cat << EOF > /var/lib/tftpboot/grml.pxe
#!ipxe

initrd http://${next-server}/grml/initrd.img
chain http://${next-server}/grml/vmlinuz boot=live fetch=http://${next-server}/grml/grml64-small.squashfs apm=power-off nomce noprompt noeject grml2ram vga=791  lang=de ssh=grml console=tty0 console=ttyS0,115200n8lang=de ssh=grml
EOF

cd /var/lib/tftpboot
ln -s grml.pxe default

Links
http://git.grml.org/?p=grml-live.git;a=blob_plain;f=templates/GRML/grml-cheatcodes.txt
http://wiki.grml.org/doku.php?id=terminalserver#grml_netboot_packages
https://www.pendrivelinux.com/vga-boot-modes-to-set-screen-resolution/