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/files/mysqld.config

@@ -0,0 +1,7 @@
+
+config mysqld 'general'
+	option enabled '0'
+	option log_stderr '1'
+	option log_stdout '1'
+	option options ''
+