scripts: runner: add network port helper class
scripts: runner: add network port helper class

Some of the flashing scripts try to be clever about picking unused
ports. That's convenient for the user, so add a helper class to
runner.core to accomplish similar ends portably.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,fbsder/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,nashif/zephyr,fbsder/zephyr,punitvara/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,aceofall/zephyr-iotos,kraj/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,explora26/zephyr,kraj/zephyr,zephyriot/zephyr,kraj/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,kraj/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,nashif/zephyr,explora26/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyriot/zephyr,fbsder/zephyr,zephyriot/zephyr,galak/zephyr,fbsder/zephyr,punitvara/zephyr,fbsder/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr