Rewrite (and relicense) the header files which are included in all
builds of iPXE (including non-Linux builds).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

There exists an smbios userspace library so implementing this is
probably possible, but doesn't seem really important to have in
userspace.  Hence provide a dummy implementation returning an error.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>