watchdog: fix init script for sysvinit

watchdog: fix init script for sysvinit

The current init script is installed from source with redhat style.
It does not get configuration from /etc/default/watchdog. We should use
debian style init script.

Write our own script just like what wd_keepalive does.
Also, in the init script, we check the existence of /dev/watchdog to
determine whether to start the daemon or not.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>