Code allowing the use of sonar without requiring a barometer.

This allows the use of sonar for altitude hold, without requiring a barometric sensor to be present.

It follows on from pull request https://github.com/hydra/cleanflight/pull/39

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 code has been flight tested on a Naze32 acro, with no barometer onboard.  It also works when the target doesn't have BARO defined.