dev-manual, mega-manual: Updated downloads figure and some links.

Found a couple broken links.  Also, noticed the yp-download figure
was out of date.  Especially in the Figures folder of the mega-manual.

(From yocto-docs rev: 2e55faca221186d157dd5f77d048f1b2b90d5d91)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>