Update board.cmake to use nrfutil for flashing

[Pferdebockwurst]

nrfjprog is deprecated and replaced by nrfutil

[rbaron]

Hey @Pferdebockwurst, thank your for the PR.

I updated the wiki page with an important note on both the SDK and toolchain versions.

If I understand it correctly, both SDK and toolchain must be set to 2.7.0 for a clean build with the current repo. At that toolchain version, nrfjprog was still the recommended way. Please correct me if I'm wrong.

We will still need this PR when we migrate to newer SDKs though. I will suggest we merge it into a 3.x.x-dev branch for now.

[Pferdebockwurst]

The current nRF Connect Extension does not include nrfjprog anymore. So you would have to install it anyway. And I don't know if it is even recommended for the SDK version 2.7.0. As far as I understand, Nordic semi replaced it with nrfutil and recommends to use that, Independently of the SDK you have built the firmware with. The need to install nrfutil seems to be a bug. It should be included in the nRF Connect Extension.