cve-check: Allow warnings to be disabled

When running CVE checks in CI we're usually not interested in warnings on the
console for any CVEs present. Add a configuration option CVE_CHECK_SHOW_WARNINGS
to allow this to be disabled (it is left enabled by default).

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 1054d336)
Signed-off-by: Steve Sakoman <steve@sakoman.com>