mysql: start at runlevel 5 not S

Without that change target system had to be restarted to get MySQL
running or it had to be run by hand. Reason was simple: mysql init
script was starting before S99configure when package was not yet
configured.

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>