libpng: use SourceForge mirror
The Gentoo mirror also deletes old versions when they're not used, so revert
back to the canonical SourceForge site, adding /older-releases/ to MIRRORS to
handle new releases moving the version we want.
Original idea by Maxin B. John <maxin.john@intel.com>.
(From OE-Core rev:
791a3493c88c9c249f21f6d893b2061e1d8a0af6)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Hand applied to work with morty version
Signed-off-by: Armin Kuster <akuster808@gmail.com>
