classes/create-spdx: Fix file:// in downloadLocation

file:// URIs should not be included as the downloadLocation. Instead,
loop until a non-file:// URI is found, or set the location to
NOASSERTION if none is found

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