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
mavlink: use altitude AMSL in VFR message
Merge branch 'master' into vfr_fix
Reduce ROMFS footprint
Reduce too chatty content, de-allocate non-needed string buffers
fix variable name in rc.uavcan
rename PWM_OUTPUTS to PWM_OUT in all files
rename DO_AUTOCONFIG to AUTOCNF in all files
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