base.bbclass: introduce OE source mirror network as last resort for do_fetch

base.bbclass: introduce OE source mirror network as last resort for do_fetch

Updating recipes and checksums.ini for vanishing upstream hosts or simple
restructuring can quickly become a chore for OE devs if taken seriously.
The OE user experience suffers severely when at any given point in time
4 to 5 recipes on average fail do_fetch for a simple image.

The solution is for OE to provide its own caching of upstream sources.
mirrors.openembedded.org is a swarm of possibly incomplete source mirrors
provided by the community and load-balanced via DNS round robin.  They
act as a first line of defense and lower bandwidth load on melo which
hosts sources.openembedded.org.  The melo mirror aims to have a complete
copy of free-to-publish sources used by the current org.oe.dev branch.

More information at http://wiki.openembedded.net/index.php/Sources_mirror

Acked-by: Holger Freyther <zecke@selfish.org>
Acked-by: Chris Larson <clarson@kergoth.com>