efi_loader: create stub for CreateEventEx

Currently we set the function pointer for the CreateEventEx boot service
to NULL. When called this would lead to an immediate failure.

A function stub is provided which handles the case that the boot service
is called without an event group and returns EFI_UNSUPPORTED otherwise.

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