add CV_CORE_PATH for optionally having rtl repo dir symlinked locally

Created by: silabs-robin

This PR adds the possibility to point a core directory in core-v-cores to a local directory via symlink. This is instead of cloning. The benefit is that one doesn't have to be afraid of some make clean or other scripts accidentally wiping the core directory. Hence one can safely work on a core's RTL and run ci_check, clean-build core-v-verif, etc.

(This is something we had locally, intermediately before the "multi-design" core-v-verif repo was ready. I am merely porting it over to the github repo now.)