This patch exposes the GRUB configuration options that control where and
how the user interface is presented. Generally, you can use it to put
the GRUB menu on a serial port, get a text-only menu on the kernel
console, or even use Morse code on the PC speaker (untested!).

You can use it like this, for example:

(bootloader (grub-configuration
            (device "/dev/sda")
            (terminal-outputs '(gfxterm console))
            (terminal-inputs '(serial console usb_keyboard))
            (serial-speed 115200)
            (serial-unit 0)))

Since this offers new ways to break your system, the patch sets up some
input validation. Hopefully it's enough.

The general idea came from this conversation:

https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00053.html

Your feedback?