ext: open-amp: Change build integration so its not recursive

With recent changes to open-amp we can now include and build it directly
as a zephyr library rather than doing a recursive make.  We remove
ext/lib/ipc/open-amp.cmake as part of this change and introduce a
Kconfig option for open-amp.

Fixes: #7673

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
10 files changed