Add remaining flight time and distance calculations and OSD items

[shellixyz]

Answers #2804 and partially #2792. Also closes #3215.

Calculates the remaining available flight time and travel distance before needing to go straight to home based on GPS distance from home, remaining battery capacity, average power draw, altitude and direction change during RTH and wind estimation. The OSD items will blink when the timer reaches 0.

New settings:

• osd_use_wind_compensation: use wind estimation to compensate for time/distance calculation ON/OFF
• idle_power: power drawn by the system when the motor(s) are stopped
• cruise_power: power drawn by the motor(s) at cruise throttle in cW (0.01W)
• nav_fw_cruise_speed: speed at cruise throttle in cm/s
• rth_energy_margin: desired energy left after arriving to home (for landing) in % of usable battery capacity

It will only display the time if the following conditions are met:

• The VBAT and CURRENT_METER features are enabled
• The GPS is healthy and the home position has been set
• The battery capacity has been specified with the mWh unit
• The cruise_power and nav_fw_cruise_speed settings have been set (>0)
• The battery was full when plugged in
• The FC is armed

Obviously be careful with this:

• It can only be as precise as the meters calibration
• Wind estimation has not yet been well tested and is not updated very often
• If not configured correctly the estimated time can be way off
• It is advised to set nav_fw_cruise_speed a bit lower than the real speed and cruise_power 10% higher than the power at cruise throttle to ensure variations in throttle during cruise won't draw more power than estimated.

[stale]

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[shellixyz]

Testing is in progress. It is working but I did some minor adjustments and need to test fly it one more time.

[teckel12]

@shellixyz Distance left in combination with distance from home could yield an automatic RTH feature.

[shellixyz]

Yes it is planned :). It is already possible with this code. The distance to home is already taken into account and soon the wind estimation from #2450.

[teckel12]

@shellixyz I don't think even DJI uses wind estimation for the auto RTH feature.

[shellixyz]

I've updated this PR. It has been tested and is ready for review/merge.

[teckel12]

@shellixyz Travis doesn't agree.

[shellixyz]

@teckel12 Now it does :)