Clone Verif Environment - Script

Created by: silabs-robin

This PR adds a script to clone the external 40x verif environment subtree repo.

It is clean, simple, and extensible to other cores.

I can rewrite this in Python or make if we think that would make things better. But I honestly think this is an uncomplicated problem and that this simple shell script is a fitting solution. @silabs-hfegran, @MikeOpenHWGroup, this continues #1867