CMakeLists: add cmake target to build Doxygen docs

adding cmake target to build Doxygen documentations. Doxygen docs are built together with the application by default if Doxygen is installed in the system, and can be disabled by disabling BUILD_DOC option. The resulting html pages are generated in the build/html directory.

Signed-off-by: Francesco Pham francesco.pham@huawei.com