load_serialno() is only ever called on a hikey by a special piece of code in BL1 that is reachable only when we don’t boot from eMMC (i.e. during the debrick proceedure).
To be honest there’s a strong argument that code like this doesn’t belong in ARM trusted firmware in the first place (fastboot could probably be implemented by the BL31 payload even when booting from USB). As such you should not assume that there is any plan to implement something similar for hikey960; even if it were implemented as a temporary convenience in the long term this type of code should be removed (otherwise we become misaligned with upstream ARM TF).