[ins] hack float_invariant to support geo_mag module

closes #858 until we have a proper solution of publishing local magnetic field via ABI messages
[ardrone2] add rm_dir command
[ardrone2] send filter status from navboard

- it allows to see if the mag is crashed or not
- set a max retry on the reset PIC board hack (which doesn't seems to
  help anyway)
[module] add agl_dist module
[math] fix polyfit init
[airframes] fix quad_navgo actuators for nps