Update submodule nuttx to latest Sun Sep 23 08:37:24 EDT 2018 (#10550)
Update submodule nuttx to latest Sun Sep 23 08:37:24 EDT 2018 (#10550)

- nuttx in PX4/Firmware (f3cbd3c744e9224bd55597fbfae23c56ce296544): https://github.com/PX4-NuttX/nuttx/commit/98a89513060c42215e9cd14f52a7fd7dcc08dacc
    - nuttx current upstream: https://github.com/PX4-NuttX/nuttx/commit/e31e94f5bd3cf68b9ed0f010724ac9122730dd21
    - Changes: https://github.com/PX4-NuttX/nuttx/compare/98a89513060c42215e9cd14f52a7fd7dcc08dacc...e31e94f5bd3cf68b9ed0f010724ac9122730dd21

    e31e94f 2018-09-21 David Sidrane - [REJECTED] kinetis:usbdev use CONFIG_KINETIS_USBOTG_PRIO at default
krbeverx/Firmware,acfloria/Firmware,dagar/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,Aerotenna/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,Aerotenna/Firmware,krbeverx/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,Aerotenna/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware