Iris: enable battery failsafe by default
Plane: fixed AUTO_FBW_STEER stick mixing
Plane: fixed throttle suppression for AUTO_FBW_STEER=1
AP_Param: explicitly print floats to 6 dec places

C++ default is to print 6 decimal places but nuttx displays none by
default
DataFlash: explicitly print floats to 6 dec places

C++ default is to print 6 decimal places but nuttx displays none by
default