mysql: version bump to 5.1.68

mysql: version bump to 5.1.68

This patch bumps MySQL to latest version and also repairs
missing/not working mirrors (planetmirror is offline and
holywar.net does not provide mysql anymore). If a source
package does not exist at mirror repo (they seem to keep
the latest two versions at mysql repo), it uses the archives
repo at last.

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@35637 3c298f89-4303-0410-b956-a3cf2f4a3e73