cve-update-db-native: let the user to drive the update interval

cve-update-db-native: let the user to drive the update interval

Add a new variable CVE_DB_UPDATE_INTERVAL allowing the user to set
the database update interval.
 - a positive value sets an interval (in seconds)
 - a zero ("0") forces the database update

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit fe7bc6f16184d5ebdb1dd914b6dcb75c9e5e0c9c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>