Just a convenient knob for a few things done previously
to help spare the interesting bits from being overrun.

NB: live_rw isn't added although it might be useful
    since that would yield too many boot targets overall;
    this is likely to change some day, hopefully when
    media type detection/handling is implemented.

Let's bump syslinux timeout to 20 seconds either
so that iKVM users have a chance to select anything
and not just see the default booting after a few
screen area size changes.