cve-update-db-native: make it possible to disable database updates

Make it possible to disable the database update completely by using
a negative update interval CVE_DB_UPDATE_INTERVAL.

Disabling the update is useful when running multiple parallel builds
when we want to have a control on the database version. This allows
coherent cve-check results without an database update for only
some of the builds.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>