In the bootloader menu, input from a USB external keyboard or a
Macbook’s built-in USB keyboard is ignored by GRUB.  This makes it
impossible to choose which generation to boot.  The passphrase of an
encrypted boot partition can be entered though (with US English
keyboard layout instead of the chosen layout).

The attached patch makes the USB keyboard work in the bootloader menu,
but it uses US English layout instead of the chosen layout too.  Also
the patch is just imitating what is already in gnu/bootloader/grub.scm
but I do not actually know what I’m doing.

Regards,
Florian