Reworked certificates handling. The certs.sh.example contains a full example of this, and must be copied to certs.sh, and modified accordingly to reflect the real paths where the certificates are stored. A script that can be called by any board is creating symlinks from the real paths to corresponding statics targets for usage in genimage and postbuild scripts. The certs folder in the external tree must never be distributed in the repository, and is hence ignored via .gitignore.