Move readme to manifest

We described most of the project specific build instructions in the
mat-guf-machine layer. This is not too convenient as the main entry
point is the manifest repository.

Adress 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: I50f2708d59ea99d865a08c6137db2d96cd1014c2