create-spdx: handle links to inaccessible locations

create-spdx: handle links to inaccessible locations

When a link is pointing to location inaccessible to build user (e.g. "/root/something"),
filepath.is_file() throws "PermissionError: [Errno 13] Permission denied".
Fix this by first checking if it is a link.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>