22.2.3 WaitForKey and WaitForKeyEx Notification Implementation
When the WaitForKey and WaitForKeyEx events are created, they must be
associated with an event notification function. This event notification
function checks to see if one or more keystrokes are currently available from
the console input device. If one or more keystrokes are currently available
from the console input device, then the WaitForKey and WaitForKeyEx events
must be placed into the signaled state by calling the UEFI Boot Service SignalEvent()
.