To make it easier to maintain the source code of the rpmsg demo, create
the the components/apps directory to host the source code, to separate
the source code from the stack integration.

At the same time of code separation, the files and directory structure
are re-arranged, and the corresponding CMake files and recipe bb files
are updated.

Issue-Id: SCM-4876
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Ia1f08b6bc88e49acb711d893ff6f2c82c4125b24