The while loops waiting ACK_WAIT_TIME and AFTER_ACK_DETECTED_WAIT_TIME
cause infinite loop under the cooja-ip64 platform. This is because
RTIMER_NOW(), rtimer_arch_now(), has been changed not to call
cooja_mt_yield() in it since rtimer was reimplemented as a higher
resolution timer.
In order to avoid the infinite loop, cooja_mt_yield() needs to be
called inside the while loops under the platform as well as the cooja
platform.
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