Add mctpreactor for dynamic configuration of MCTP networks

While mctpd[1] may see heavy use in projects such as OpenBMC, it
implements generic functionality necessary to operate MCTP as a
protocol.

Add mctpreactor for dynamic configuration of MCTP networks

While mctpd[1] may see heavy use in projects such as OpenBMC, it
implements generic functionality necessary to operate MCTP as a
protocol. It therefore should be easy to use in other contexts, and so
it feels unwise to embed OpenBMC-specific details in its implementation.

Conversely, entity-manager's scope is to expose inventory and board
configuration. It externalises all other responsibilities for the sake
of stability and maintenance. While entity-manager is central to
OpenBMC's implementation and has little use in other contexts, embedding
details of how to configure mctpd in entity-manager exceeds its scope.

Thus we reach the design point of mctpreactor, an intermediary process
that encapsulates OpenBMC-specific and mctpd-specific behaviors to
constrain their dispersion in either direction. The design-point was
reached via discussion at [2].

mctpreactor tracks instances of transport-specific MCTP device
configurations[3] appearing as a result of inventory changes, and uses
them to assign endpoint IDs via mctpd.

The lifecycle of an MCTP device can be quite dynamic - mctpd provides
behaviors to recover[4] or remove endpoints from the network. Their
presence cannot be assumed. mctpreactor handles these events: If
a device is removed at the MCTP layer (as it may be unresponsive),
mctpreactor will periodically attempt to re-establish it as an endpoint
so long as the associated configuration on the entity-manager inventory
object remains exposed.

[1]: https://github.com/CodeConstruct/mctp/
[2]: https://github.com/CodeConstruct/mctp/pull/17
[3]: https://gerrit.openbmc.org/c/openbmc/entity-manager/+/70628
[4]: https://github.com/CodeConstruct/mctp/blob/7ec2f8daa3a8948066390aee621d6afa03f6ecd9/docs/endpoint-recovery.md

Change-Id: I5e362cf6e5ce80ce282bab48d912a1038003e236
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

show more ...