firmware: prevent writing or opening during verification
firmware: prevent writing or opening during verification

If the current state is during verification, prevent opening blobs or
writing to blobs.

Change-Id: Ia903e52a5f8f8adaeb4372b850f798e61141d9b6
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>

openbmc/phosphor-ipmi-flash