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="unknown"

 reporter_url="http://iphonelinux.org/index.php/Glibc_toolchain_using_crosstool-NG"

 reporter_comment="An (experimental) toolchain targeting the Apple's iPhone."