scripts: validate SPDX license choices

scripts: validate SPDX license choices

We expect all new code to be contributed with the "GPL-2.0-or-later"
license tag. Divergence is permitted if the new file is derived from
pre-existing code under a different license, whether from elsewhere
in QEMU codebase, or outside.

Issue a warning if the declared license is not "GPL-2.0-or-later",
and an error if the license is not one of the handful of the
expected licenses to prevent unintended proliferation. The warning
asks users to explain their unusual choice of license in the commit
message.

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