x86: when building image.iso, use isohybrid if it exists

When building image.iso (make isoimage), use the isohybrid tool if it
exists.  isohybrid is a script included with Syslinux 3.72 and higher,
which creates an image that can be booted either as a hard disk
(including removable, e.g. USB disk) or as a CD-ROM.

If isohybrid doesn't exist, then this has no effect.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>