mariadb: update /var/run to /run

The following warning has been occuring with mariadb, update the
references to use the non legacy directory - /run

systemd[1]: /storage/.config/system.d/service.mariadb.service:8:
  PIDFile= references a path below legacy directory /var/run/,
  updating /var/run/mysqld/mysqld.pid → /run/mysqld/mysqld.pid;
  please update the unit file accordingly.

packages/addons/service/mariadb/source/bin/mariadb.start

@@ -7,7 +7,7 @@
 oe_setup_addon service.mariadb
 
 # create dir for socket and pid
-mkdir -p /var/run/mysqld
+mkdir -p /run/mysqld
 
 # exit if already running
 PID=$(ps aux | awk '/\/bin\/mariadbd/ {print $1; exit 0}')