README: Move README.md description to manifest
We described most of the project specific build instructions in the mat-seconorth-machine layer. This is not too convenient as the main entry point is the manifest repository.
Address that by moving the instructions to this manifest repository and later removing the description from the meta-guf-machine layer. Adapt the SECO name change here as well.
BCS 746-000659
Change-Id: Ifabe294b58101b40246f75442feed305e07f2921