Move Inventory.Item.Board from Redfish Chassis to Redfish Assembly

[Gunnar Mills]

 From some discussion on Discord a few weeks ago. A newer way of 
thinking in Redfish is for a simple rack server to just have one Redfish 
Chassis and things like boards modeled as Redfish Assemblies under the 
Chassis.[1][2] Today we look for inventory item chassis and inventory 
item board[3] to populate the Chassis collection.[4] I propose we move 
the "Inventory.Item.Board" from Redfish Chassis to Redfish Assembly. 
Does this break anyone? Does anyone have an inventory item board that 
can't be a Redfish Assembly?

Another option would be to only move certain inventory item boards to 
Redfish Assembly, determined by a BoardType or additional more detailed 
Inventory Item interfaces around boards. Unless this is needed though 
would prefer the former for simplicity.

https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/39574 is adding 
the Assembly schema. This would be a later commit moving 
"Inventory.Item.Board".

[1]https://redfish.dmtf.org/redfish/mockups/v1/1018
[2]https://redfishforum.com/thread/325/fru-properties-redfish

[3]https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Inventory/Item/Board.interface.yaml

[4]https://github.com/openbmc/bmcweb/blob/8d1b46d7f8d39db2ba048f9e9007106ca3a28c9b/redfish-core/lib/chassis.hpp#L189

https://redfish.dmtf.org/schemas/v1/Chassis.v1_15_0.json
https://redfish.dmtf.org/schemas/v1/Assembly.v1_3_0.json

Thanks,
Gunnar