mariadb: fix warning while invoking 'mysql_install_db' on target

Invoke 'mysql_install_db' on target, there are some warnings:
...
Installing MariaDB/MySQL system tables...
131009  6:13:14 [Warning] '--skip-locking' is deprecated and will be removed in a future release. Please use '--skip-external-locking' instead.
OK
Filling help tables...
131009  6:13:14 [Warning] '--skip-locking' is deprecated and will be removed in a future release. Please use '--skip-external-locking' instead.
OK
...

Modify my.cnf to use '--skip-external-locking' instead of '--skip-locking'
could fix this issue.

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>