I was using Ubuntu 18 Server install, but I was able to get it working like this.
I created a /images/os/ubuntu folder and copied the ubuntu ISO file contents in there.
I created a folder /tftpboot/os/ubuntu
There I put the initrd and vmlinuz files from the casper folder in the ubuntu iso.
Here are the parameters I used in FOG.
imgargs vmlinuz initrd=initrd boot=casper root=/dev/nfs netboot=nfs nfsroot=192.168.160.12:/images/os/ubuntu/ ip=dhcp rw
boot || goto MENU
This allowed me to start the installer from that usb, but I haven’t got a working live OS running yet, though I hope to have that soon.