optionally wait for device available before erase

This is mainly for streamlining testing in certain secanrio:
* build is applied to devices outside of TF
* TF bench is started
* test invocation would wait for device to be ready, instead
  of waiting outside of TF for all devices to be ready

Change-Id: I91983c98864a16c509d46dbe00943e57523b35e5
1 file changed