bmc: use json for configuration
bmc: use json for configuration

Use a json file for configuration of the flash image.

Tested: Verified that /flash/image worked as expected.
Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>
Change-Id: I205978203360140870f0e68208912a894e00a6fc

openbmc/phosphor-ipmi-flash