issue (GCC 6.1.1):
../src/modules/fw_pos_control_l1/fw_pos_control_l1_main.cpp:1284:27: error: implicit conversion from ‘float’ to ‘double’ to match other operand of binary expression [-Werror=double-promotion]
  if ((fabs(air_gnd_angle) > M_PI) || (ground_speed_2d.length() < 3.0f)) {

Report terrain altitude

If the circuit breaker to disable IO safety is engaged, the safety
button should still blink in the appropriate pattern: double flash for
safety off, solid on for armed.

Since the FRAM and the baro are on the same bus on the Pixracer, we
currently need to lock down everything (instead of just this SPI bus)
for the time when the params are written.
Therefore, we need to keep this locking as short as possible.

This change makes the locking even shorter by moving all param_get and
param_name and param_size calls out of the lock.

Previously, the accel DLPF was not set, so it's not clear what the
settings are or should be.

This is a big refactor and general cleanup of the land detector. The
main functional change is to share the hysteresis lib across all land
detectors.

Since the auto disarm hysteresis was implemented wrongly, it's now
replaced with the hysteresis library call.

The result from `!get_freefall_state()` was always true because
`get_freefall_state()` is already called before `get_landed_state()` is
called. Only if we're not in a freefall, we check if we are landed.

The hysteresis was not properly implemented in the land detector and is
therefore replaced by the library call, both for the freefall detector
and the land detector.

Sometimes, we only need a histeresis in one direction.

There have been two cases where a hysteresis function wasn't working
correctly. It is therefore a good idea to abstract the hysteresis
functionality into a library.