Add configuration files for running RF-A tests
This PR adds configuration to build and run RF-A (v0.1.0) with its tests.
This needed changes to extract most of tfa-base.yaml into tfa-root.yaml in order to select a different BL31 than the one from TF-A. I made separate commits to keep git lineage of tfa-root.yaml content (i.e. git blame will do the right thing).
I have included some rationale about the strategy used by rust.yaml for installing the Rust toolchain. Currently, rustup and cargo currently have some behavior that does not mesh very with Docker images and will result in many cases in superfluous downloads.