Rework the entropy interface a bit. Move from entropy_efi_seed to
entropy_seed to enable this (future boot loaders will be able to get
croptographicly secure seeding). Check to see if we have the
'seed-entropy' loader function. If we do, then we'll use that and seed
the entropy if either entropy_seed or entropy_efi_seed is
'yes'. Otherwise, if we don't have a newer loader, then use the old way
of only calling efi-seed-entropy if we're running on UEFI.
Add entropy_seed to default/loader.conf and add documentaiton that it's
the new one to use.
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