random-seed: refresh EFI boot seed when writing a new seed

random-seed: refresh EFI boot seed when writing a new seed

Since this runs at shutdown to write a new seed, we should also keep the
bootloader's seed maximally fresh by doing the same. So we follow the
same pattern - hash some new random bytes with the old seed to make a
new seed. We let this fail without warning, because it's just an
opportunistic thing. If the user happens to have set up the random seed
with bootctl, and the RNG is initialized, then things should be fine. If
not, we create a new seed if systemd-boot is in use. And if not, then we
just don't do anything.