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/toolchain-build/action.yml

@@ -0,0 +1,29 @@
+name: toolchain-build
+description: 'Builds the selected toolchain'
+
+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: toolchain-build-id
+      with:
+        path: ${{ inputs.tools-version }}-install
+        key: ${{ inputs.cache-key }}
+    - name: run build toolchain if not cached
+      run: |
+        if [[ "${{ steps.toolchain-build-id.outputs.cache-hit }}" != 'true' ]]; then
+          echo "Cache miss on ${{ inputs.tools-version }}-install"
+          ./.github/scripts/build-toolchains.sh ${{ inputs.tools-version }}
+        else
+          echo "cache hit do not generate build ${{ inputs.tools-version }}"
+        fi
+      shell: bash
+