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: NH. Peter Anvin <hpa@zytor.com>

arch/x86/boot/Makefile

@@ -181,6 +181,7 @@ isoimage: $(BOOTIMAGE)
 	mkisofs -J -r -o $(obj)/image.iso -b isolinux.bin -c boot.cat \
 		-no-emul-boot -boot-load-size 4 -boot-info-table \
 		$(obj)/isoimage
+	isohybrid $(obj)/image.iso 2>/dev/null || true
 	rm -rf $(obj)/isoimage
 
 zlilo: $(BOOTIMAGE)