Here is a config I use for PXE booting both UEFI and legacy BIOS clients at the same time.

This is the GRUB variant; for the previous version I used see the pxelinux branch.

The main config is in boot/grub/grub.cfg

How to use

1. Run ./make.sh on Ubuntu to copy the required files from system packages
2. Set up a TFTP server to serve files from the ./tftp directory
3. Set up a HTTP server to serve files from the ./tftp directory (because TFTP is horribly slow, and you would need to load the initrd over it otherwise, which is ~100MB on recent Ubuntu releases)
4. Set up an NFS server to serve files from the ./tftp/disks directory (you may be able to get by with just HTTP in newer releases of Ubuntu if you have enough RAM to fit the entire ISO)
5. Set up a DHCP server to provide correct config file depending on the type of the client. See dnsmasq.conf for raw dnsmasq, or etc_config_network for OpenWRT
6. Configure boot/grub/grub.cfg to your needs. An example of booting Ubuntu 20.04.3 Installer/Live CD is provided.