Update submodule ecl to latest Fri Jan 25 00:38:20 UTC 2019

    - ecl in PX4/Firmware (b12ddf8): PX4/PX4-ECL@acde4eb
    - ecl current upstream: PX4/PX4-ECL@721f5e6
    - Changes: PX4/PX4-ECL@acde4eb...721f5e6

    721f5e6 2018-12-18 Anna Dai - increase optical flow buffer to imu buffer length
44200e9 2018-12-18 Anna Dai - add GPS drop out case to GPS fusion logic
8a813c5 2019-01-10 CarlOlsson - EKF: fix comment typo
472f228 2019-01-10 CarlOlsson - EKF: initialize covariances before we reset the heading in order to preserve the yaw uncertainty
4908882 2019-01-08 CarlOlsson - EKF: Remove flag prefix
d223680 2019-01-04 CarlOlsson - EKF: add flt_mag_align_complete to control_status flags
b920910 2019-01-06 Daniel Agar - replace <cfloat> with <float.h>
c032126 2019-01-04 Paul Riseborough - EKF: Fix bug causing incorrect yaw variance value to be used
48d83f3 2019-01-04 Paul Riseborough - EKF: Fix vulnerability to ground level mag anomally when using heading fusion (#544)
4822bf8 2018-12-07 Hamish Willee - Fix links to moved files in readme
1a969ae 2018-11-21 CarlOlsson - EKF: use low pass filtered mag measurements for heading initialization
27f7846 2018-12-02 Roman - optical flow: fixed sign in calculation of optical flow sensor velocity
c49ab76 2018-06-20 Paul Riseborough - EKF: Allow EKF to start without using mag data when not using earth frame data
572710e 2018-11-14 Daniel Agar - EKF add typed Quatf getter
8a2e512 2018-06-28 CarlOlsson - terrain_estimator: add vehicle_variance_scaler