OPENTHREAD_CONFIG_CSL_TRANSMIT_TIME_AHEAD macro

@def OPENTHREAD_CONFIG_CSL_TRANSMIT_TIME_AHEAD Transmission scheduling and ramp up time needed for the CSL transmitter to be ready, in units of microseconds. This time must include at least the radio's turnaround time between end of CCA and start of preamble transmission. To avoid early CSL transmission it also must not be configured higher than the actual scheduling and ramp up time.