scripts/checkpatch: reimplement mandate for SPDX-License-Identifier

scripts/checkpatch: reimplement mandate for SPDX-License-Identifier

Going forward we want all newly created source files to have an
SPDX-License-Identifier tag present.

Initially mandate this for C, Python, Perl, Shell source files,
as well as JSON (QAPI) and Makefiles, while encouraging users
to consider it for other file types.

The new attempt at detecting missing SPDX-License-Identifier relies
on the hooks for relying triggering logic at the end of scanning a
new file in the diff.

Tested-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>