Zephyr board porting

• https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.3.2/zephyr/guides/porting/board_porting.html#create-your-board-directory

Likely we will move this nRF9160 datasheet reference:

• https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf9160%2Fpin.html

Here see section titled 'Example standalone application':

• https://docs.zephyrproject.org/latest/application/index.html