CMake: improved dependency handling using add_subdirectory_once macro

CMake: improved dependency handling using add_subdirectory_once macro

each component now just builds all components it dependes on using
the custom macro 'add_subdirectory_once'.

During the first call, add_subdirectory_once will add the subdirectory
to the current build. all subsequent calls are silently ignored.
This solves the basic problem of double definition of components which
result in cmake errors.

Signed-off-by: Timo Lotterbach <timo.lotterbach@bmw-carit.de>