fix nativenet_send documentation
Merge pull request #472 from LudwigOrtmann/nativenet_send

fix nativenet_send documentation
mute build tools (to be consistent with the rest of the build system)
build everything in the project directory
move include directives from header to C file
simplify and unify include pathes

additional:
* exporting include path in sys is mandatory for subfolders
* removed duplicate object file in linker call
correct filename
simplify include structure for boards

* do not overwrite CFLAGS in boards Makefile
simplified and unified cpu build structure
removed redundant include pathes from Makefiles
removed superfluous slash after BINDIR