create-spdx: handle CLOSED license

The special CLOSED license means that this is closed source code with
no other licensing assertions.  It's not a generic license in oe-core,
and not a SPDX license, so transform it to NONE explicitly.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>