scripts/download: add sources CDN as first mirror

scripts/download: add sources CDN as first mirror

OpenWrt now has a CDN for sources at sources.cdn.openwrt.org which
mirrors sources.openwrt.org.

Downloading sources outside Europe or US (mainland) could
result in low throughput, extremely slowing down the first compilation of
the build system.

This patch adds sources.cdn.openwrt.org as the first mirror to offer
worldwide fast download speeds by default. If the CDN goes down for
whatever reason, the script jumps to the next available mirror and
downloads requested files as before (in regional varying speed).

Signed-off-by: Paul Spooren <mail@aparcar.org>
Acked-by: Eneas U de Queiroz <cotequeiroz@gmail.com>