efi_loader: implement event groups

If an event of a group event is signaled all other events of the same
group are signaled too.

Function efi_signal_event is renamed to efi_queue_event.
A new function efi_signal_event is introduced that checks if an event
belongs to a group and than signals all events of the group.
Event group notifciation is implemented for ExitBootServices,
InstallConfigurationTable, and ResetSystem.

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