at_keyboard flush on i386-qemu

[Robert Millan]

Hi,

Pavel pointed out earlier that sometimes when starting on i386-qemu GRUB
receives spurious events from AT keyboard.  It seems that it is the role
of the firmware to flush this buffer at startup.

Unless someone has a better idea, I would fix this with:

  - Moving at_keyboard to kernel on i386-qemu.

  - [ifdef GRUB_MACHINE_QEMU]: flush the input buffer at at_keyboard
    startup by reading and discarding events for a fixed amount of time.

I don't like that we have a race here.  Suggestions welcome on how to
improve that, but TTBOMK if there's more than one event you can't tell
when you're processing the last one.

-- 
Robert Millan

  The DRM opt-in fallacy: "Your data belongs to us. We will decide when (and
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