net: openthread: Tx started notification
net: openthread: Tx started notification

OpenThread uses "tx started" notification to start ACK
timeout if the driver does not support it.
In order for this to be achieved tx task needs to run in parallel
to the OpenThread task, otherwise it may delay the gap between
cca and tx or run after transmission has already finished.

Signed-off-by: Marek Porwisz <5823cd812b1ea828ac01e8b54624ab4f83fea720@nordicsemi.no>

zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr