Add flag to only allow TPM commands via IPMI.

This flag will be enabled for Bloom machines only. For full details, please see go/hoth-tpm-ipmi-blob.

Testing

A bmc image was created and installed on a Bloom test machine. Then,
non-TPM commands were sent from the host through the haventool_lite
tool, which failed:

`ipmi_send_command failed: 0xFF`

In contrast, and as expected, commands sent through the tpmtool
succeeded and measurements were returned. The full testing logs are here: https://paste.googleplex.com/5173528069537792

Google-Bug-Id: 473880547
Change-Id: Id684d46744b3a30de9d5f083f0a1306a1107c863