efi_loader: implement task priority level (TPL)

Define variable holding tpl.
Implement RaiseTPL and RestoreTPL.
Implement TPL check in efi_signal_event.
Implement TPL check in efi_wait_for_event.

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