package/php: switch from libmysqlclient to mysqlnd

package/php: switch from libmysqlclient to mysqlnd

The Mysql Native Driver has been the default mysql driver since
PHP 5.4, but buildroot was still using libmysqlclient.

Mysqlnd has several advantages such as improved memory management
and the more favorable PHP licensing terms.
(can combine it with proprietary PHP extensions like Ioncube
loader, while libmysqlclient requires commercial licensing if you
link to it and do not fall under their GPL/FOSS license exception)

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>