create-spdx: Set the Organization field via a variable

Currently, the "Organization" field for SBOMs is hard-coded in
create-spdx. Create a new variable SPDX_ORG to make this field more
generic.

Signed-off-by: Andres Beltran <abeltran@linux.microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>