Merge remote-tracking branch 'upstream/master' into airspeed

src/modules/commander/accelerometer_calibration.c

src/modules/commander/accelerometer_calibration.h

...modules/fixedwing_att_control/fixedwing_att_control_att.c

...odules/fixedwing_att_control/fixedwing_att_control_rate.c

...odules/fixedwing_pos_control/fixedwing_pos_control_main.c

...ules/multirotor_att_control/multirotor_attitude_control.c

...ules/multirotor_att_control/multirotor_attitude_control.h

src/modules/multirotor_att_control/multirotor_rate_control.c

src/modules/multirotor_att_control/multirotor_rate_control.h

src/modules/sdlog2/logbuffer.c

src/modules/sdlog2/logbuffer.h

src/modules/sdlog2/sdlog2.c

src/modules/sdlog2/sdlog2_format.h

src/modules/sdlog2/sdlog2_messages.h

src/modules/systemlib/pid/pid.c

src/modules/systemlib/pid/pid.h

src/modules/uORB/topics/optical_flow.h