* mc_pos_control: ignore a NaN/inf position setpoint

This is a hotfix that prevents the position controller from trying to
do velocity control with a position setpoint that isn't valid in the
first place.

This is only a workaround, ideally the controls later should not scale
down throttle to the minimum just because the position setpoint is far
away if they still have valid setpoint in z.

* mc_pos_control: use PX4_ISFINITE and not isfinite

The free memory was accessed from interrupt context where it should not be accessed from. We build the statistic now at a fixed rate while not armed.

and avoid doing a loop iteration when the thread is requested to exit
(as it could access _buffer).

 - Use the course over ground as loiter exit criteria (better wind behavior)
 - Compute the tangent origin coordinate explicitly instead of using immediate position

IO needs atomic updates of a few critical status flags, but doing these always atomic (with interrupts disabled) might cause a too high interrupts disabled interval. In order to avoid this only operations that change the state of the target variable are done with interrupts disabled, while operations without an effect on the target variable are not executed.

This enables to use the RAM normally consumed by the log buffer to be used for calibration and other memory-intense tasks.
These run typically only disarmed when logging is not enabled.

Initialisation changes to address valgrind errors
Change to default GPS and Airspeed time delay (these are overwritten by ekf2_main parameter settings)
Increase sensitivity of bad accelerometer checks

The learned ekf2 mag bias values are invalidated when the sensor calibration is updated and must be reset.

Squashed commits:
[290660d] voted_sensors_update: revert the new_accel_data check (and others)

The check removed the ability to detect sensor timeouts.
[c8dc7ad] sensors: publish changes to sensor selection
[dd90dec] sensors: ensure all sensor selections published first time

Move into functions.
Reset probation time and recalculate checks if a mode change is demanded to give the operator ability to regain control as soon as possible after nav performance is regained. (+11 squashed commits)
Squashed commits:
[a4bb800] commander: enable pilot to quickly recover from loss of position accuracy
[19e16a0] commander: rework postal probation time
[f96284e] commander: rework bad pos and vel test probation time
[00d5f0c] commander: Allow EKF preflight checks to pass with moving vehicle

Separates the 'is using GPS' and the GPS quality checks.
Uses a reasonable subset of the GPS quality checks which allows checks to pass if the vehicle is moving.
[4cdfb5c] commander: remove unused variable
[349385a] commander: add EKF GPS quality checks to pre-arm checking

Only perform check if GPs checking is activated by parameter setting.
Display fault messages that makes it clear if EKF quality checks are failing or the EKF is not using GPS for another reason. We do not want to confuse this with GPS lock.
[340ae29] commander: make position invalid fail-safe more sticky

Require check to pass for 7 seconds before exiting failsafe. This is required because if GPs is failing innovation tests for a prolonged period, the EKF will periodically reset to the GPS and report good accuracy at the time of reset.
Adding this delay gives time for an underlying error condition (eg bad IMU or compass) to be re-detected.
[b04ac95] commander: Increase RAM allocation to eliminate low stack warnings
[9dca12f] commander: add missing position invalid fail-safe responses
[69f264d] commander: Update position invalid fail-safe responses

Replace separate logic for each case with a generic function
Add velocity checks.
[8e8cef1] commander: rework position validity checks

Consolidate existing checks for global and local position validity and add checking of velocity accuracy.
Enable checks to be bypassed using the CBRK_VELPOSERR parameter.

Add missing velocity accuracy reporting
Add missing dead reckoning reporting

   Added board_on_reset, to force timer GPIO to output low on reset
   This will stop the motors.

  This optionally provided function called on entry to
  board_system_reset. It should perform any house keeping
  prior to retunring to do the rest.

  When the CCER is cleared the IO pin tends to float. The FMUV4
  HW has no TXS0108 and if cut off while high will decay.

  By adding the pull down the pins will seek the low state faster.
  13.8 us from off to decabe below threshold.