Feature sonar flight mode

This allows the use of sonar for altitude hold, without requiring a barometric sensor to be present.

A new box is created in the Auxiliary Configuration page, which allows the user to configure aux switches to enable or disable sonar mode.

This system still needs work, as it hasn't been tested above the sonar range limit (I'm not sure what will happen there).  But it has been tested and shown to work, at least in a proof of concept sense.

![screen shot 2014-08-13 at 9 39](https://cloud.githubusercontent.com/assets/489642/3900350/13db43f8-228b-11e4-9fe1-5c05a9b9808c.png)