buildroot: tee-supplicant: use -d (daemonize) option and print OK/FAIL

Init scripts normally print OK or FAIL when they start daemons. Update
our tee-supplicant script to do just that. The -d option to tee-supplicant
is used so that when the tee-supplicant command returns, we  know for sure
if initialization went well or not.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>