support/scripts/pkg-stats: iterate over CVEs in streaming

support/scripts/pkg-stats: iterate over CVEs in streaming

The NVD files that are used to build the list of CVEs affecting
Buildroot packages are quite large (a few hundreds MB of json),
and cause the pkg-stats scripts to have a huge memory footprint
(a few GB with Python 2.7).

However, because we only need to iterate on CVE items one by one,
we can process them in streaming (ie decoding one CVE at a time
from the JSON representation). Because the json module from the
python standard library does not support such a mode of operation,
we switch to the third-party package ijson, which is compatible
with both Python 2 and Python3.

To run the script with these modifications, one should install
the ijson python package. This can be done with pip:
`pip install ijson`. On Debian based distributions, this can
also be done with the apt package manager:
`apt install python-ijson`.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Reviewed-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Tested-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>