My specific goal is about a dynamic OS bootstrapping or via USB
device or via pci netcard automatically without pressing any keyboard
seabios skips any non-working entries, a blank disk for example. So you
can set the bootorder to "disk first, net second" and as long as no os
is installed seabios will try to pxeboot, after install it'll boot from
the disk instead. Which might be dynamic enough for your use case ...