I think you can get that by setting `GRUB_TIMEOUT_STYLE=hidden` and `GRUB_TIMEOUT=0`. Then you can hold `Shift` to see the GRUB menu, otherwise it will boot the default option immediately.
Make sure this works on your system before you really need it, as some keyboard/USB-controller combinations take a few seconds to wake up from whatever slumber they're in and start working. I ran into this several times.
