Synchronise with NetBSD's version of EFI handling for El Torito images.

When I implemented my EFI support I failed to check if the upstream version
of makefs in NetBSD had done the same. Override my version with theirs to
make it easier to stay in sync with them in the future.

Reviewed by: imp, mav
Obtained from: NetBSD
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14913