Vendorize CVA6 core submodules based on flist PR #1005

Created by: zchamski

This PR implements the vendorization of submodules under common and core subdirectories of CVA6. It supersedes the initial vendorization PR #1006 and takes into account the comments associated with that PR.

Main features:

• All configuration files are located under vendor.
• Naming of configuration files follows OpenTitan vendorization conventions, with extra suffixes added to <project_name> to distinguish between independent instances of the same project (cf. PULP Platform's common_cells).
• Imported submodules are located under vendor/<vendor_name>/<project_name> or appropriately placed inside the enclosing project (cf. submodules of PULP Platform's fpnew)
• Makefile and the Flist.* files of the CVA6 project were adjusted to point to the vendorized locations.
• The original submodules of CVA6 common and core are retained for the time being, because no change was made so far to the UVM testbench references in core-v-verif.

Signed-off-by: Zbigniew Chamski zbigniew.chamski@thalesgroup.com