[syslinux] Install gptmbr with Windows

[Ady Ady]

> Oh somehow I never noticed that "bootsecfile" option. That's probably what
> I need to use. I was looking at the source to see how the --mbr flag works
> and thought that there should be a way to do the same thing with the other
> mbr files.
> 

No, the [bootsecfile] option is not the answer to your issue.

KISS: use "dd for Windows".

If you want / need more than just writing gptmbr.bin to your USB device 
under Windows, then simply use some auxiliary tool such as RUFUS, or 
RMPrepUSB, or bootice, or...

The available options for the different Syslinux installers should 
rather keep converging, instead of adding more special cases for each.

Regards,
Ady.