config: hide paths when used as a backend

When crosstool-NG is used as a backend, it is the responsibility of the
upper-layer build-system to properly set paths, so we just hide the
prompts in this case.

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""