Log message:
efivar: add latest changes from wip

The ONLY_FOR_PLATFORM was tightened, on my own request. This will teach
me to "git pull" before copying over stuff from wip :(

PR pkg/57559 still

Log message:
devel/efivar: Add efivar 38

This is an adaptation of https://github.com/rhboot/efivar to make it
work on BSD systems.  Amount of changes is quite substantial and is
very similar across *BSD systems, so a fork is used instead of a set of
patches (there are still small patches in the port related to building
on NetBSD).

Fully implementing libefiboot for a system will take a lot of work, so
it has limitations [1].  Generating abbreviated device paths works and
that's enough to boot an EFI payload, making it useful for fwupd.

[1]: https://github.com/3mdeb/efivar/blob/bsd/BSD_PORTING.md

Patch provided by Sergii Dmytruk in PR pkg/57559.