The meaning of the yaw weight changed with #8003:
- before, the yaw weight decreased with increasing tilt angle error, so
  it was mostly 1
- now, it is constant and depends on the tuning gains (around 0.4 by default)

It means that #8003 reduced the feedforward term, and we get the closer
behavior as before with this change.
It also reduces coupling between different parameters.

memset (http://www.cplusplus.com/reference/cstring/memset/) interprets
the value argument as unsigned char, so passing a float will not work.

The description of IMU_GYRO_CUTOFF was incorrectly saying that it only
affects the driver filtering, but in fact it also set the on-chip filter
to the next higher supported value. This patch fixes that.

And because the IMU_GYRO_CUTOFF and not the IMU_ACCEL_CUTOFF was used for
the on-chip filter, after #9070 which sets the default gyro filter to 80,
we were effectively using a dlpf of 98 Hz.
For this reason this patch changes the on-chip cutoff frequency to 98 as
well, so that the overall default behavior is unchanged.

 - EKF2 airspeed fusion and synthetic sideslip fusion
 - commander eph/epv failsafe thresholds appropirate for FW

    - Changes: https://github.com/PX4-NuttX/nuttx/compare/fe0dc6c41a6bd7d942945459ac960ff2d3e1f4fc...65b87d092a6d5f776ab5f9dbb095d31e45861789

    65b87d0 2018-04-19 Daniel Agar - [PX4 BACKPORT] Merged in dagar/nuttx/pr-stm32f777_typo (pull request #628)

- useful for tuning snapdragon flight

Signed-off-by: Roman <bapstroman@gmail.com>

 * ECL update includes physical constants for https://github.com/PX4/Firmware/pull/9319

Signed-off-by: CarlOlsson <carlolsson.co@gmail.com>

Signed-off-by: CarlOlsson <carlolsson.co@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>

- reduce height increase during front transition

Signed-off-by: Roman <bapstroman@gmail.com>

- the mc pos controller will decrease throttle during the transition
and thus the vehicle will not pick up enough airspeed to complete the
transition

Signed-off-by: Roman <bapstroman@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>

- removing unused variables
- removing unused code blocks
- improving code readability
- initialising members at definition