classes/create-spdx: Add packageSupplier field

Per NIST SBoM recommendations, include the Package Supplier field for
all SPDX packages that are created. This field should generally be set
to the person or organization that is performing the build, since they
would be considered the "supplier" of the SPDX packages.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>