exclude-list: generalise exclude-list

exclude-list: generalise exclude-list

Currently exclude-list.json is used by the xen-analysis tool to
remove from the report (cppcheck for now) violations from the
files listed in it, however that list can be used by different
users that might want to exclude some of the files from their
computation for many reasons.

So add a new field that can be part of each entry to link
the tool supposed to consider that exclusion.

Update exclusion_file_list.py to implement the logic and update
the documentation to reflect this change.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>