efi_loader: event signaling in ExitBootServices

ExitBootServices() has to stop timer related activity before calling the
events of the EFI_EVENT_GROUP_EXIT_BOOT_SERVICES event group. But our
current implementation was stopping all other events.

All events have to observe the task priority level.

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