mariadb: install config files readable for all

mariadb: install config files readable for all

Until now $(INSTALL_CONF) is used for configuration files that go into
/etc/mysql. This commit replaces that with $(INSTALL_DATA). The
configuration files are not only parsed by the server, but also by the
clients (which can be anybody).

This also removes a comment about a cron job from one of the
configuration files. There is no cron job.

Signed-off-by: Sebastian Kemper <redacted>