phosphor-host-ipmid entity map moving from YAML to JSON

[Patrick Venture]

If you provide your own entity YAML file for phosphor-host-ipmid:
https://github.com/openbmc/phosphor-host-ipmid/blob/master/scripts/entity-example.yaml

Previously, this was done via a virtual provider, but that was dropped
some time ago.  Now you'd replace that file to have the effect.

Well, if you're one of these machines you now will be moving to JSON
at run-time.  There'll be a file to add to phosphor-ipmi-config named:
"entity-map.json"

The schema for the json is similar to the YAML and is documented here:
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-host-ipmid/+/24551/scripts/entity-example.md

Nobody upstream is using this -- no system that is merged into
openbmc/openbmc.  I was using it before downstream.  Perhaps others
are too :)

Patrick