random(4): Abstract loader entropy injection

Break random_harvestq_prime up into some logical subroutines. The goal
is that it becomes easier to add other early entropy sources.

While here, drop pre-12.0 compatibility logic. loader default configuration
should preload the file as expeced since 12.0.

Approved by: csprng(delphij, markm)
Differential Revision: https://reviews.freebsd.org/D22482