Better GPS accuracy reading for FrSky telemetry

[teckel12]

Previously, the GPS accuracy value being sent to FrSky telemetry was basically useless as an HDOP 10 or less was considered the highest accuracy. Basically, this would cause the GPS accuracy to read either 0, the worst, or 9 the best.

With this change, the GPS accuracy is more granular. A value of 9 (the best) is now when the HDOP is 2.0 or less. Each step down for the accuracy value is an HDOP of 0.5 more. So an 8 is from 2.0 to 2.5, 7 us from 2.5 to 3.0, etc. with a 0 being an HDOP higher than 6.0.

This change allows for a more useful reporting of the GPS fix accuracy, better informing the pilot of any potential navigation issues.

[fiam]

@teckel12 Since you're probably the only one using the FrSky telemetry as an API, I think this is safe to merge as soon as it's working for you. Just let us know when it's ready.

[teckel12]

@fiam Tested a few times, ready to merge.

I didn't change other telemetry protocols that use the same old useless HDOP logic. But my guess is that few if any are looking at it anyway.