loader: Load platform entropy even without menu

In 5c73b3e0a3db calls to core.loadEntropy were added to core.boot
and core.autoboot; but neither of those is invoked if we disable
the "beastie" menu. Add a core.loadEntropy call to the no-menu
path.

Reviewed by: imp
MFC after: 1 week
Sponsored by: Amazon
Fixes: 5c73b3e0a3db ("Add support for getting early entropy from UEFI")
Differential Revision: https://reviews.freebsd.org/D46637

(cherry picked from commit 74a28cf6e7f66c7c12fd25ee8231eeedf756bf08)