mysql: add mysql init script

[Thomas: explicitly specify the path to the PID file, and also on the
first boot, generate the mandatory MySQL system tables so that the
MySQL daemon actually starts properly.]

Signed-off-by: Marcelo Gutiérrez <kuyurix@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

merge init script