[doc] correct doc in board makefiles to change default actuators
[imu] aspirin: no default SMPLRT_DIV for non-default GYRO_LOWPASS"
[imu_umarim] configurable gyro lowpass and output rate
[fixedwing] define default AHRS_PROPAGATE_FREQUENCY and AHRS_CORRECT_FREQUENCY in code instead of makefiles
[imu_umarim] set default AHRS_PROPAGATE_FREQUENCY to 50Hz in makefile
[fixedwing] print used ahrs freqs, pass configure options on...
[airborne] only print config messages at compile time if PRINT_CONFIG is defined