Make IPMI OEM commands optional feature

[Kostr]

Currently phosphor-pid-control requires phosphor-host-ipmid for the build, to support IPMI OEM command functionality (ipmi.md).

It would be a nice to have this functionality as an optional feature opposed to the necessary requirement, since phosphor-pid-control can be used on the systems without IPMI stack.

[Krellan]

Agreed. A patch is welcome, to implement the feature of making IPMI optional.