[syslinux] Add support for Variables

[Ryan McLean]

   Hmm  this  is perhaps where my lack of C / assembly knowledge will let
   me down but here goes.
   PXE downloads pxelinux to the client, parsing the conf file.
   so  the  conf  file  is  in memory, could we not set aside memory (say
   enough  for  32  chars, we can use CNAMEs to shorten the http server's
   name  to  fit that, as most of us run the dns server too :) ) and copy
   the var there if it exists.
   on  loading a new conf file that area of memory is checked for a value
   which  is then passed to the menu ( like a macro in C is substituted )
   if  the  "var"  appears again later on in the  conf  it will overwrite
   the  section of memory that was previously used (say deleting that mem
   loc and then re using it, or using the string term char (\0) to end it
   if it is shorter than the previous conf's var.)
   As  i say i have no idea if the above is feasible as it has been years
   since  i even looked at assembly and i am only really starting to look
   at C / C++ again.
   Regards,
   Ryan McLean