costmap_2d: fix radius warning when footprint is set

If the footprint parameter is set, inscribed and circumscribed radii are
computed from that. However, the check was done *before* that computation
and therefore used the default params. This commit

- moves the check after the computation
- changes the wording from "inscribed and circumscribed radii" to
 "inscribed or circumscribed radius".

Also see
[this](http://answers.ros.org/question/62277/inscribed-and-circumscribed-radii-of-the-robot-used-despite-footprint-being-set/)
ros answers thread.