make the definition of external/box2d as an override more robust

The osdep overrides are only used when necessary, so this all
lets-explicitly-check-for-OS-version-and-name stuff is really
not needed, as already explained in #2
define drivers/orogen/kinect unconditionally

Autoproj will automatically exclude it on architectures where
drivers/freenect won't be available. This commit also makes sure that
the (idiotic) freenect versioning requirements are showing up in
the osdeps