[syslinux] Add support for Variables

[H. Peter Anvin]

Ryan McLean wrote:
> Was just looking at a number of my scripts ( due to having to update them 
> for RH5).
> 
> Would it be possible to add support for variables / 2 variables 
> 

Common request, although different people want different things with it. 
  It's not all that obvious what the Right Thing[TM] to do is; it's also 
not obvious if this can be done, realistically, in a way that is 
compatible with the CLI console.

	-hpa