[util] Inclusion of OpenTitan's vendor util

OpenTitan's vendor util can be used to rebuild or update the external dependencies under vendor/ by feeding it with the .hjson files located on such directory.

It may be used in the future if we want to add or update external dependencies, also allowing the eventual removal of their contents on vendor/*/*, instead of having to obtain this script manually from OpenTitan's repository.

This is just a copy of it where I added an example as a comment.