Investigate best way to integrate multiple repos

[nathalie-ckc]

• Arnaud can try to modify his CI to try submodule, to see how easy to work with it

• Bill will experiment Zephyr integration idea with West

(Filing GH Issues for relevant action items from System Reference calls. From https://github.com/OpenAMP/openamp-system-reference/wiki/OpenAMP-System-Reference-Working-Group-Meeting-Notes#user-content-Action_items-4)

[wmamills]

@wmamills and @arnopo have defined a method using west and a new repo.
This is the approach we will use for Zephyr.
I suggest we use it for all contents so we don't have repo for FreeRTOS and submodules for bare-metel etc.
west is easy to use w/o zephyr. We can supply different manifest in the root of openamp-system-reference for different contexts.