Purpose

Improve usability of VPTOOL, for now on CVA6 verification plans:

[generic] Print all elements of a verification item in human-readable form, including the lists of applicable cores;
[generic] Improve the selection and handling of markdown output directory;
[CV32A6] Generate MD output for all items applicable to CV32A6-stage1, including those applicable to other cores;
[CV32A6] Improve robustness of FRONTEND verification plan editing script;
[CV32A6] Regenerate FRONTEND verif plan using updated VPTOOL.

Change log

cva6/docs/VerfiPlans/source/dvplan_FRONTEND.md: Regenerate.
tools/vptool/vptool/vp_pack.py (Item.attrval2str): New. (Item.__str__): Convert enumerated and bitmapped fields to associated plain text strings using labels from Yaml file.
tools/vptool/vptool/vp.py (MyMain.save_db): Send MD output to directory source, sibling of the database directory. Skip generation of MD output altogether if <database_dir>/../source designates a plain file. If source does not exist and cannot be created, send MD output to <database_dir>. Update comments. Generate item description in MD format if the list of applicable cores includes CVA6-stage1.
cva6/docs/VerifPlans/FRONTEND/runme.sh (ROOTDIR): Use path from bash arg list. Update comment. (PYTHONPATH): Use correct directory and keep previous paths if any. (vptool invocation): Update comment.