optee services: update script

Update script based on the feedback from Buildroot maintainer [1]:

 > +# /etc/init.d/optee

 Drop this comment, it is useless, and in fact wrong: the file will not
 have this name in a Buildroot filesystem.

 (...)
 > +	if [ -e /usr/sbin/tee-supplicant -a -e /dev/teepriv0 ]; then

 Drop this test, just start tee-supplicatn.

 (...)
 > +		echo "Starting tee-supplicant..."
 > +		/usr/sbin/tee-supplicant &

 Please use start-stop-daemon. See
 https://patchwork.ozlabs.org/patch/994013/ for the "right" way of
 writing an init script.

This maybe not be best way, if there's one, but it seems more
standard to use start-stop-daemon. Moreover the proposed script
ensures its return value matches the status of the caller request.

[1] http://lists.busybox.net/pipermail/buildroot/2018-December/238354.html



Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>