support/scripts/pkg-stats: add support for CVE reporting

support/scripts/pkg-stats: add support for CVE reporting

This commit extends the pkg-stats script to grab information about the
CVEs affecting the Buildroot packages.

To do so, it downloads the NVD database from
https://nvd.nist.gov/vuln/data-feeds in JSON format, and processes the
JSON file to determine which of our packages is affected by which
CVE. The information is then displayed in both the HTML output and the
JSON output of pkg-stats.

To use this feature, you have to pass the new --nvd-path option,
pointing to a writable directory where pkg-stats will store the NVD
database. If the local database is less than 24 hours old, it will not
re-download it. If it is more than 24 hours old, it will re-download
only the files that have really been updated by upstream NVD.

Packages can use the newly introduced <pkg>_IGNORE_CVES variable to
tell pkg-stats that some CVEs should be ignored: it can be because a
patch we have is fixing the CVE, or because the CVE doesn't apply in
our case.

>From an implementation point of view:

 - A new class CVE implement most of the required functionalities:
   - Downloading the yearly NVD files
   - Reading and extracting relevant data from these files
   - Matching Packages against a CVE

 - The statistics are extended with the total number of CVEs, and the
   total number of packages that have at least one CVE pending.

 - The HTML output is extended with these new details. There are no
   changes to the code generating the JSON output because the existing
   code is smart enough to automatically expose the new information.

This development is a collective effort with Titouan Christophe
<titouan.christophe@railnova.eu> and Thomas De Schampheleire
<thomas.de_schampheleire@nokia.com>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>