scripts: when downloading, use aria2 only when //

Even when // downloads are not enabled, aria2 can
fail on some servers (eg. uclibc.org).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Harold Grovesteen"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

 reporter_comment=""