Every file should be installed exactly once by `make installworld`.
This is especially important for pkgbase.
Loader help files were being installed by each loader variant (e.g.,
the simp, lua, and 4th EFI loaders). Add a (slightly hacky) mechanism
to skip installing help files for all but one variant.
PR: 271178
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40021
e32fecd0c2