9dof: Use real world values instead of raw ints.

The sensors are the only ones knowing the units they measure so
change the API to return real world units (e.g. degrees/second)
instead of raw 16 bit integer values that mean nothing outside
the driver code.
9dof: Add possibility to calibrate gyroscope.

Assume steady initial state, calculate the average gyroscope offset
and remove the offset from the subsequent readings sent to main
application.
user thread: Remove forgotten header.
9dof: Fix header guard.
9dof: Store a real timestamp with the values.
ahrs: Allow GPL code.
ahrs: Add AHRS ready event handling.
ahrs: Add AHRS module without actual algorithm.

Add a new module for launching a separate thread for running the AHRS
algorithm. The algorithm will contain heavy math and is running in a
separate thread with lower priority to avoid increasing latencies for
reset of the system.
lis302dl: Fix initial semphore value.
lsm9ds0: Minor fixes and cleanup.
utils: Add sc_ftoa(). Fix identation.
conf: Disable AHRS by default.
event: Increase workarea due to AHRS.
build: Add AHRS builds.
9dof: Add warmup and calibration.
ahrs: Add GPL C++ AHRS implementation.

Upstream:
https://sourceforge.net/projects/ce10dofahrs/

uimu_ahrs_glue code provides glue code between the C and C++ codes so
that the AHRS files could be just copied from the upstream without
any changes for easy maintainability. Currently two includes need to
be changed but no actual code changes are needed.