OPENTHREAD_CONFIG_FRAME_TX_ERR_RATE_AVERAGING_WINDOW macro

@def OPENTHREAD_CONFIG_FRAME_TX_ERR_RATE_AVERAGING_WINDOW OpenThread's MAC implementation maintains the average error rate of MAC frame transmissions per neighbor. This parameter specifies the window (in terms of number of frames/sample) over which the average error rate is maintained. Practically, the average value can be considered as the percentage of failed (no ack) MAC frame transmissions over (approximately) last AVERAGING_WINDOW frame transmission attempts to a specific neighbor.