update attitude_estimator_ekf, includes matlab

This adds the latest c implementation (matlab coder) of
attitude_estimator_ekf, the .m matlab script and the .prj file with the
settings to export the matlab code to c
Merge remote-tracking branch 'upstream/master' into attitudeekf
att ekf: add param to enable/disable J
att ekf: add descriptions for params
Merge remote-tracking branch 'upstream/master' into attitudeekf
Merge remote-tracking branch 'upstream/master' into attitudeekf
Merge remote-tracking branch 'upstream/master' into attitudeekf
update attitude estimator ekf to latest version

mainly saves stack size
Implemented altitude and velocity hold mode
Corrected time_gps_usec values description.

Fixes #1474
fixed somereview comments
use consistent orientation naming in messages
swap fron/back > the "side" being measured is facing downwards
move natural position to the front of the pending list for QGC
Merge pull request #1479 from AndreasAntener/accel-calibration-fix

Accel calibration fix
Merge pull request #1475 from hsteinhaus/gps_time

Corrected time_gps_usec values description.
Merge master into attitude EKF update
Display ESC data in the status output
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Merge pull request #1482 from sjwilks/uavcan_debug

Display ESC data in the status output