nRF52840DK Board Definition for the Tock Dynamic Apps and Secure Policies Tutorial

This is the board definition for the nRF52840DK target used in the [Dynamic App Loading and Secure Policies] (https://github.com/tock/book/pull/60/files).

Please follow the instructions in that tutorial. You may also want to look at the documentation of the base nRF52840DK board definition here.