MFC r331463 (partial), r331467, r331468, r331843

r331463, r331467, and r331468 are all replaced by r331843 but are included for
completeness' sake. r331463 also contained a change to
release/amd64/mkisoimages.sh which I've left out since those changes were
also obsoleted by a later commit which will be merged later.

r331843:

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

Sponsored by: iXsystems, Inc.