cve-update-nvd2-native: Add an age threshold for incremental update

cve-update-nvd2-native: Add an age threshold for incremental update

Add a new variable "CVE_DB_INCR_UPDATE_AGE_THRES", which can be used to
specify the maximum age of the database for doing an incremental update
For older databases, a full re-download is done.

With a value of "0", this forces a full-redownload.

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 74c1765111b6610348eae4b7e41d7045ce58ef86)
Signed-off-by: Steve Sakoman <steve@sakoman.com>