efi_loader: add check_tpl parameter to efi_signal_event

In ExitBootServices we need to signal events irrespective of the current
TPL level. A new parameter check_tpl is added to efi_signal_event().

Function efi_console_timer_notify() gets some comments.

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