According to the document[1], we shouldn't block unknown cr50 phase in PVT.  But currently /usr/share/cros/cr50-set-board-id.sh will just exit with error said "Board ID and/or flag has been set DIFFERENTLY on Cr50!" when we want to set "PVT" phase while the phase is already "unknown".  Then the factory toolkit will just raise exception in this case.

We should either
1) Modify the toolkit logic to pass such case.
2) Modify the cr50-set-board-id.sh script to pass such case.


Also, we might need to show warning message if we found that the phase is already set to "unknown".


[1] https://docs.google.com/document/d/1--8hUw8awtQy6ci5rQrSO3MwFJAzc0SlXLmuDOWjsno/edit#