Official Ubuntu ISO should use isomd5sum for embedded checksum

[Kip Warner]

The Ubuntu ISO and official derivatives could contain embedded MD5 checksums at the iso9660 metadata layer by using isomd5sum tools to implant and check the checksum. This may be faster than checking each package at the file system level because the verification process when done at the block level is a single linear seek without having to jump around. The checksum can even be verified after the image has been burned and mounted.

From the tool's README:

    isomd5sum provides a way of making use of the ISO9660 application data
    area to store md5sum data about the iso. This allows you to check the
    iso given nothing more than the iso itself.