- create directory /etc/mysql/conf.d as without it the server refuses to
  start
- correct the path to my.cnf in the init script

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

MariaDB is a drop-in replacement for MySQL. This commit adds a current
and stable version of MariaDB to the tree.

Quite a few ideas/patches were copied from Alpine Linux, Busybox Buildroot
and Debian.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>