Until this is "fixed" by the Pi project (indeed, most PCs will boot loop if no boot devices succeed), I'd try something like iPXE (loaded onto SD or USB.) That

Until this is "fixed" by the Pi project (indeed, most PCs will boot loop if no boot devices succeed), I'd try something like iPXE (loaded onto SD or USB.) That should be able to loop properly. https://github.com/ipxe/pipxe https://github.com/ipxe/pipxe