mysql: make it work after $libexec change - LP: #1068759

Mysql start scripts assumed that mysqld is in /usr/libexec/ directory.
It was true until 406bd38b4232f9f399ef5ffe0b4fac72ed605a23 landed in
OE-Core.

This changeset fixes mysqld start and initial databases population
scripts.

More: https://bugs.launchpad.net/linaro-oe/+bug/1068759



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