multiboot2: remove "a.out kludge" requirement

[Hollis Blanchard]

One requirement listed for multiboot2 was that ELF headers should be
ignored, because NetBSD was setting their ELF addresses incorrectly.
This has now been fixed (see
http://www.netbsd.org/cgi-bin/query-pr-single.pl?number=32865), so I've
removed that requirement.

ELF headers tell you where to load an executable far better than the
kludge, so it really doesn't make sense to ignore ELF.

-Hollis