scripts: correctly warn when the local tarballs directory does not exist

Warn about a missing local tarball directory, only if it was configured.
Avoid the spurious message:
Directory '${CT_LOCAL_TARBALLS_DIR}' does not exist.
Will not save downloaded tarballs to local storage.

Thanks to "Pedro I. Sanchez" <psanchez@colcan.ca> for pointing out the
issue:
http://sourceware.org/ml/crossgcc/2009-12/msg00011.html

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/debug.in

 source config/tools.in

 source config/companion_libs.in