Extend the RPMsg structs to accommodate for the introduction of new
backends and contextually fix the ipc_rpmsg_static_vrings_mi backend
(the only user).
Rework also some comments and ipc_service glue code.
Signed-off-by: Carlo Caione <ccaione@baylibre.com>
The IPC libraries will be used by several backends. Move the libraries
out in a new 'lib' directory.
Signed-off-by: Carlo Caione <ccaione@baylibre.com>