boards: mps2_an521: no empty CPU0 binary for openamp applications
boards: mps2_an521: no empty CPU0 binary for openamp applications

When we are building applications with openAMP on CPU1, we do not
need to trigger a build an empty binary for CPU0, as this will be
take care of by the dual core sample anyways.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

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