shell-utils: Integrate shell-utils into libweston

Patch series integrates shell-utils into libweston and exports them for other shells to use. While at it, add a few sphinx entries for shell-utils and for weston-config and mention it in the libweston main rST page. Adds also a two group names such that they can be browsed, but without any documentation added at this point.

Finally, this moves out one of functions from shell-utils to weston-config as that seems a more suitable place rather then keeping it in shell-utils.

There's no functional change.

I've been thinking about getting shell-utils into their own libweston-util library but just for the shell-utils that didn't seem worth introducing a library at this point, but if that seems more reasonable than keeping these into libweston I'd fine with that as well.