support/scripts/pkg-stats: URL checking support

support/scripts/pkg-stats: URL checking support

- Adds support to check if a package has a URL and if that URL
  is valid by doing a header request.
- Reports this information as part of the generated html output

The URL data is currently gathered from the URL string provided
in the Kconfig help sections for each package.

This check helps ensure the URLs are valid and can be used
for other scripting purposes as the product's home site/URL.
CPE XML generation is an example of a case that could use this
product URL as part of an automated update generation script.

CC: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>