flightgear HIL: also use default Scenery directory besides terrasync

Warnings fixes

Add libudev-dev to linux build requirements

This removes a race condition between both qwt and qt against math.h on
windows with _USE_MATH_DEFINES set. If math.h comes after qwt or qt
math headers the math defines will be redefined. Really nasty to track
down and ordering problems. So instead I just removed using math
defines on windows and fall back to asin definition of M_PI_2. This
will have some slight perf impact since it calls function instead of
using constant. But I doubt on any of today’s machine it really makes a
difference.

Number of warnings is low enough now to let them come through. Lowered
Windows warning level the /W3 to be more consistent with OSX/Linux
level of warnings. Removed /WX from Windows build. I’ll put it back
once everything is cleaned up.

Conflicts:
	src/comm/QGCFlightGearLink.cc

QGC depends on libudev-dev to build, include this in the list of packages to install.

Airframe config: fixed hexa copy and paste error

PX4 config: Remove the specialized throttle handling - this needs testing, but doing the math on paper it doesn’t look like a special case is needed / wanted here.

Use the reversed values for the four control channels as well. Not using them would make the display very confusing.

Enable RC calibration mode during RC calibration to tell vehicle to ignore stick inputs for that period

Added more generated files to the .gitignore.

Updated Linux build info in readme

Remember to init submodules for "qupgrade"

Even more warnings

Reorder warnings