Does the HiKey board have a unique ID in the SoC that can be read out and that allows identifying different board in software (e.g. in UBoot or from Linux). Anything that is quasi-fixed in the hardware would be nice, e.g. a serial number in ROM or fuses that can be read directly, but a unique root key in a peripheral that can be used to generate something would also work.
Putting something into the flash is a fallback solution, as this can be changed easily. Where does the MAC of the board come from?