scripts: protect against partially extracted source

In case of failure (eg. user hitting Ctrl-C), avoid extracting over
partially extracted source. Will help once we have git available.

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="s390x"

     fi

 }