With the help of the iPXE community, I found a solution. I copied RtkUsbUndiDxe.efi and ipxe.efi (from the uefi-pxe.zip file in the
Jaguardboard instructions ) to blk0:\EFI\BOOT\. Then I created a startup.nsh file in the same directory with these contents:
load blk0:\EFI\BOOT\RtkUsbUndiDxe.efi
ipxe.efi
The system performs an automatic netboot just how I wanted.