Custom nRF52840 board and GNU ARM G++

I just found PlatformIO, and want to use it with a custom nRF52840 board using the nRF_sdk, C++, and FreeRTOS. I am having difficulty figuring out how to create a custom board. I have been looking over Custom Embedded Boards but it is not clear to proceed.

Has anyone else successfully defined a custom nRF52 board using the nRF_sdk and GNU ARM GCC or do I need to create a custom platform to accomplish this?