Get bootable ISO image functional.

This required making paths to images in syslinux config APPEND line
absolute, as well as adding mboot.c32 to the isolinux dir.
Disable hddimg.

My IvyBridge systems complain about them having 'invalid partition table!'
but my SandyBridge systems think they're fine.  I don't even think there
is a partition table on the hddimg so maybe my new Dell is more sensitive
to this than my older systems?

Regardless isos can be cat'd out to usb drives anyway so they can be used
/ tested in nearly the exact same way.  Just can't modify them directly
which you probably shouldn't do anyway.