cve-update-nvd2-native: Handle BB_NO_NETWORK and missing db

cve-update-nvd2-native: Handle BB_NO_NETWORK and missing db

The custom do_fetch routine is ignoring BB_NO_NETWORK, add a check for this
as the correct behavior for the user is to set:
  CVE_DB_UPDATE_INTERVAL = "-1"

If CVE_DB_UPDATE_INTERNAL is set to -1, check that a DB file exists, if not
we need to error so the user can deal with this.

Note, MIRRORs are NOT handled by this code.

Signed-off-by: Mark Hatle <mark.hatle@amd.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>