scripts/functions: add a save/restore handler

Saving and restoring the steps requires saving/restoring multiple
directories. Depending on the configuration, some may not exist.

Add a wrapper that checks before creating/extracting the tarballs.

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

    if [ "${CT_ARCH_64}" = "y" ]; then

        CT_TARGET_ARCH="s390x"

    fi

}