Meta-boot2qt integration script cleanup for easier partner re-use

Current way of integrating new hardware targets to meta-boot2qt expect all changes to be done in meta-boot2qt alone: https://doc.qt.io/QtForDeviceCreation/qtee-adding-new-device-configurations.html. This works fine for the device that we look after with ongoing builds and tests. However, for 3rd party vendors it may be better approach to have the integration part in their own layer(s) and only use meta-boot2qt layer as is, without any HW specific changes there.

For this to happen easier, the current initialization and setup scripts would need to be restructured to allow easier integration.