Create libefivar library. This library aims to provide
the same API as the GPL'd version of this library. It implements the common
Linux API for programatically manipulating UEFI environment varibales using
the UEFI Runtime Services the kernel provides. It replaces the old efi
library since it is programmed to a different interface, but retails the
CHAR16 to UTF-8 and vice versa conversion routines. The new name is to match
Linux program's expectations.

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