Add support to wait for GPS fix.

[jah2235]

The VN-200 GPS takes about a minute to acquire a fix. It might be nice to have an option to wait to start logging until we get the fix.

For most SYSID/PID flight testing we don't need the GPS LLA, but the NED velocities might be useful, so this should be optional.

Note that a wait_for_gps option is currently available in the configuration file, but is not used.

GitHub Enterprise Issue 6