Work perfectly also with UEFI PXE boot! 
menuentry 'Ubuntu 20.04 Liveserver' --id ubuntu-server_1804 {
linuxefi /ubuntu_2004_amd64_netinstall/vmlinuz ethdevice-timeout=30 ip=dhcp url=http://ubuntudev.tux.lan/ubuntu-20.04-live-server-amd64.iso boot=casper ide=nodma debian-installer/language=de console-setup/layoutcode?=de
initrdefi /ubuntu_2004_amd64_netinstall/initrd
}