Update submodule ecl to latest Wed May 8 12:38:58 UTC 2019 (#11972)

    - ecl in PX4/Firmware (fa1fbed24633aef7985a128ebd4d19a7f6a4d108): https://github.com/PX4/ecl/commit/c4492b17c1d7bb9b74011efa9890452b091421e8
    - ecl current upstream: https://github.com/PX4/ecl/commit/a27a43eafa8f4dd514e89984f5394260a36ea4f6
    - Changes: https://github.com/PX4/ecl/compare/c4492b17c1d7bb9b74011efa9890452b091421e8...a27a43eafa8f4dd514e89984f5394260a36ea4f6

a27a43e 2019-05-01 Hamish Willee - Fix up link to EKF docs
3f69189 2019-04-04 Mohammed Kabir - EKF: control: stop vision yaw fusion on timeout

ecl @ a27a43ea

-Subproject commit c4492b17c1d7bb9b74011efa9890452b091421e8
+Subproject commit a27a43eafa8f4dd514e89984f5394260a36ea4f6