efi_loader: implement queueing of the notification function

For the correct implementation of the task priority level (TPL)
calling the notification function must be queued.

Add a status field 'queued' to events.

In function efi_signal_event set status queued if a notification
function exists and reset it after we have called the function.
A later patch will add a check of the TPL here.

In efi_create_event and efi_close_event unset the queued status.

In function efi_wait_for_event and efi_check_event
queue the notification function.

In efi_timer_check call the efi_notify_event
if the status queued is set.
For all timer events set status signaled.

In efi_console_timer_notify set the signaled state of the
WaitForKey event.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>