Move Chipyard CI to Github Actions

- As similar as possible to the circle ci code.
- The `.github/README.md` file has a fair amount of documentation for this.
- Creates a worfklow file
  - re-uses most of the circleci/scipts unchanged
  - defines a number of *Composite Actions* which are like YML subroutines
- Removes the circle-ci code
- Points the CI badge in the top level README to use the GA test result

.circleci/build-extra-tests.sh

@@ -1,11 +0,0 @@
-#!/bin/bash
-
-# turn echo on and error on earliest command
-set -ex
-
-# get shared variables
-SCRIPT_DIR="$( cd "$( dirname "$0" )" && pwd )"
-source $SCRIPT_DIR/defaults.sh
-
-make -C $LOCAL_CHIPYARD_DIR/tests clean
-make -C $LOCAL_CHIPYARD_DIR/tests