scripts/kernel_doc.py: better handle exported symbols

scripts/kernel_doc.py: better handle exported symbols

Change the logic which detects internal/external symbols in a way
that we can re-use it when calling via Sphinx extension.

While here, remove an unused self.config var and let it clearer
that self.config variables are read-only. This helps to allow
handling multiple times in parallel if ever needed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/6a69ba8d2b7ee6a6427abb53e60d09bd4d3565ee.1744106242.git.mchehab+huawei@kernel.org