[messages] dist_home and dist_wp in meters instead of squared

also add dist_home for ROTORCRAFT_NAV_STATUS
[conf] change papgets layout for dist_home
[rotorcraft] add dist_wp
[tmtc] rotorcraft_server: backwards compatibility for dist_wp
Merge pull request #784 from paparazzi/nav_message

[messages] add dist_home and dist_wp for rotorcrafts and change it to meters instead of squared.

Since dist_wp  is used on the ground to compute an approximate ETA (using distance to target waypoint and current speed) the threshold to use that information is now to 0.5m distance and 0.5 m/s speed to not get bogus ETAs when hovering with the quad.