Move Zephyr examples into Zephyr upstream

[glneo]

The Zephyr project seems open to examples and already has some for OpenAMP[0].

Examples here as a downstream will have the normal issues downstream projects tend to have (maintenance of keeping things in sync means we have these demos locked to a specific Zephyr version. Upstream they would be migrated with changes in Zephyr for any given version).

We should move the two current Zephyr examples into upstream Zephyr and update the docs (examples/zephyr/README.md) to explain how to find and build those examples in the Zephyr tree instead.

[0] https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/subsys/ipc

[arnopo]

The rpmsg_multi_services sample relies on two things that prevent it from being mergeable in its current state in Zephyr:

• It requires a patch in Linux: Linux Patch, which allows the creation of rpmsg channels from the Linux side. This patch is mandatory for the demo3.2: create a new TTY channel

• The associated demo uses the rpmsg-utils tools.

If we address these points, in my opinion, we will have a sample very close to what is already available at Zephyr openamp_rsc_table.

This would mean:

• Remove Linux patch and capability to use the NS annoucement on Linux rpmsg endpoint creation.
• removing the rpmsg_multi_services sample (and possibly improving the Zephyr openamp_rsc_table sample)
• keeping the readme file explaining the demo using rpmsg-utils.