I did not check when purchasing nodes but RockPI-X Small Board Computers (SBC) are not PXE network boot capable. Forums on Radxa (RockPi-X manufacturer) suggest to use iPXE to circumvent this limitation.
As RockPI-X use SoC, storage is not reachable from an other computer. Therefore for first installation a USB key with iPXE should be used. There is not so much documentation about iPXE, after all it is a tool for niche usage, or more precisely, it is focused on use on rackable servers, not SBC. The side effect is that I found that iPXE is a much more powerful tool, able to handle complex configurations, than I expected initially.
Finally, I managed to get information on how to deploy iPXE on a bootable UEFI USB key, thanks to the following sites:
In the case of the RockPI-X, the right iPXE firmware to use is rtl8168.efi.
Now, let’s try to provision the nodes …