Since the commit 005b6aba89eaf1b79fdd7565dd028fdd9bbfcc7d
(efivar: add musl libc compatibility) efibootmgr compiles with
musl too. Update the variable to take that into account.

(From OE-Core rev: ada5e64a97d5f269886772540e0bb0c324088b21)
Signed-off-by: Marta Rybczynska <marta.rybczynska@linaro.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>