efi_loader: implement event queue

Up to now we have only been using a flag queued for events. But this does
not satisfy the requirements of the UEFI spec. Events must be notified in
the sequence of decreasing TPL level and within a TPL level in the sequence
of signaling.

Implement a queue for signaled events.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>