frudevice: add i2c bus blacklist json

The fru-device daemon contained a black list internally of buses to not
rescan in the future. This list was generated automatically by talking

frudevice: add i2c bus blacklist json

The fru-device daemon contained a black list internally of buses to not
rescan in the future. This list was generated automatically by talking
to each bus. On a system where scanning the devices on a specific bus
can cause problems, extend that blacklist to be something that can be
specified for a machine.

This adds a file blacklist.json, which currently contains one field. An
array of bus integers named "buses." This file can be overwritten by a
platform recipe to drop in a list of buses to avoid scanning.

Tested: Verified that an empty, but valid json file (the default file
has no effect).
Tested: Verified that the array with an invalid type of entry is caught,
reported and the program exited.
Tested: Verified that the array with three valid entries prevents the
daemon from scanning the devices on those buses.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I1c426ebed2f7ac073ee45ed8b205e4a176a519ab

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