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

.github/actions/build-extra-tests/action.yml

@@ -0,0 +1,28 @@
+name: build-extra-tests
+description: 'Builds extra test required for some flows'
+
+inputs:
+  tools-version:
+    description: Which toolchain to build
+    required: false
+    default: 'riscv-tools'
+  cache-key:
+    description: Use this for caching
+    required: true
+runs:
+  using: "composite"
+  steps:
+    - uses: actions/cache@v2
+      id: build-extra-tools-cache
+      with:
+        path: extra-tests-install
+        key: ${{ needs.make-keys.outputs.extra-tests-cache-key }}
+        restore-keys: ${{ needs.make-keys.outputs.extra-tests-cache-key }}
+    - name: Build extra tests if not cached
+      run: |
+        export RISCV="/__w/chipyard/chipyard/riscv-tools-install"
+        export LD_LIBRARY_PATH="$RISCV/lib"
+        export PATH="$RISCV/bin:$PATH"
+        .github/scripts/build-extra-tests.sh
+      shell: bash
+