[syslinux] Install gptmbr with Windows

[Erik Sandberg]

Does anyone know how to install the gptmbr.bin onto a USB drive with
Windows? I noticed that it would be really easy to modify the source of the
installer to install gptmbr.bin instead of mbr.bin with the -m flag, but
I'd like a solution that doesn't require me to maintain my own custom
syslinux. I'm open to writing and submitting a patch to add a new flag (say
-g) to install gptmbr.bin but I'd really like a solution that's more like
running `cat gptmbr.bin > /dev/sdb` but on Windows.

- Erik Sandberg