random_harvestq_prime() loads any saved early entropy, if available, but
this may not always be available on e.g., embedded systems, installers,
or systems that do not use loader(8) and cannot access the /boot filesystem
Fast random sources, like RDRAND on x86 and DARN on Power are loaded as
modules earlier in mi_startup (SI_SUB_KLD). If we did not find enough saved
early entropy to unblock the random device but we do happen to have fast
random sources available, use them to provide sufficient early seeding.
If no entropy and no such sources are available, we now produce an explicit
warning to that effect.