FrSky telemetry updates

[bkueng]

This brings some updates to the frsky telemetry driver:

• fix the heading information (D protocol)
• send flight mode & gps information
• remove static's, refactor allocations to free up several 100 bytes.

There's a really cool script that can be used on the Taranis: http://ilihack.github.io/LuaPilot_Taranis_Telemetry/

It shows:

• gps lock information
• current flight mode (very useful to know whether you actually are in position mode)
• battery fill level
• global heading (if it's there, it means ekf has properly initialized the position)
• signal strength
• and more

This is almost a replacement for QGC :D

Tested on Pixracer with both D and S.Port protocols.

[LorenzMeier]

Awesome! Could you put up the documentation for it as well?

And I presume that the flight modes need a custom version of the script to show correctly?

[bkueng]

Yep, I'll do that. It's quite simple and extremely helpful, so I'm a bit surprised that no one else used that before.

The PX4 modes were already there, but I made some updates as well: ilihack/LuaPilot_Taranis_Telemetry#7

[bkueng]

PR for documentation is here: PX4/PX4-user_guide#45
Merging this.