yocto: Introduce meta-kronos layer and Kas configs

Major changes:

* Bootstrap meta-kronos layer using bitbake-layer create-layer.
* Intial Kas configurations in base.yml to extend the Kronos Kas file
  in the meta-arm layer using meta-kronos.
* Introduce baremetal.yml for image build (target is unset yet).
* Introduce ci.yml for image build in CI.

Changelog: other
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>