Making sleep time in isCmdEmpty configurable through controller config

This is a small patch to the current stable YARR + felix-star release v1.5.1 to make the sleep time in the isCmdEmpty() of FelixTxCore configurable through controller. This way users can change it easily from the controller config and can make the configuration step faster, if there are no hidden latencies in the setup. This merge request resolves the YARR issue: #236 (closed)