mariadb: update init script to use uci

Does away with /etc/default/mysqld, introduces uci configuration
instead. The init script receives some further brushing up, like a
function (copied from Debian) to get mysqld configuration parameters
easily and quickly.

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

utils/mariadb/conf/mysqld.default

@@ -1,10 +0,0 @@
-# The user and group that will run the MySQL server. The user mariadb is
-# automatically created by the mariadb-server package, hence that is the
-# default choice.
-
-#MY_USER=mariadb
-#MY_GROUP=mariadb
-
-# Additional arguments you want to pass to the MySQL server.
-
-#MY_ARGS=""