Update submodule ecl to latest Sat Sep  1 18:41:45 UTC 2018
Update submodule ecl to latest Sat Sep  1 18:41:45 UTC 2018

    - ecl in PX4/Firmware (e421c254e2f02ef5ede53b0752f66e0e7b8c6d24): https://github.com/PX4/ecl/commit/3c74dc2ce1002db5b68a833fdd0aa964210e91f5
    - ecl current upstream: https://github.com/PX4/ecl/commit/b861594d0b2f8c476cb758f9ae22c83e14920dfe
    - Changes: https://github.com/PX4/ecl/compare/3c74dc2ce1002db5b68a833fdd0aa964210e91f5...b861594d0b2f8c476cb758f9ae22c83e14920dfe

    b861594 2018-09-01 Paul Riseborough - EKF:  enable optical flow use to bootstrap

mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,Aerotenna/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,Aerotenna/Firmware,acfloria/Firmware,Aerotenna/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,Aerotenna/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware