support/scripts/pkg-stats: add support for json output

support/scripts/pkg-stats: add support for json output

Pkg-stats is a great script that get a lot of interesting info from
buildroot packages. Unfortunately it is currently designed to output a
static HTML page only. While this is great to include on the
buildroot's website, the HTML is not designed to be easily parsable and
thus it is difficult to reuse it in other scripts.

This patch provide a new option to output a JSON file in addition to the
HTML one.

The old 'output' option has been renamed to 'html' to distinguish from
the new 'json' option.

Signed-off-by: Victor Huesca <victor.huesca@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>