Add efivar(1) to manipulate EFI variables. It uses a similar command
line interface to the Linux program, as well as adding a number of
useful features to make using it in shell scripts easier (since we
don't have a filesystem to fall back on interacting with).

Differential Revision: https://reviews.freebsd.org/D8128
Reviewed by: kib@, wblock@, Ganael Laplanche