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/prepare-rtl/action.yml

@@ -0,0 +1,41 @@
+name: prepare-rtl
+description: 'Builds RTL based on parameters, caches the entire chipyard root dir when done'
+
+inputs:
+  tools-version:
+    description: Which toolchain to build
+    required: false
+    default: 'riscv-tools'
+  group-key:
+    description: group key
+    required: true
+  build-script:
+    description: rtl build script to use
+    required: false
+    default: "do-rtl-build.sh"
+  build-type:
+    description: type of build
+    required: false
+    default: "sim"
+
+runs:
+  using: "composite"
+  steps:
+    - uses: actions/cache@v2
+      id: rtl-build-id
+      with:
+        path: |
+          sims/verilator
+          sims/firesim/sim
+          generators/gemmini/software/gemmini-rocc-tests
+        key: ${{ inputs.group-key }}-${{ github.ref }}-${{ github.sha }}
+    - name: run rtl build script if not cached
+      run: |
+        if [[ "${{ steps.rtl-build-id.outputs.cache-hit }}" != 'true' ]]; then
+          echo "Cache miss on ${{ inputs.group-key }}-${{ github.ref }}-${{ github.sha }}"
+          ./.github/scripts/${{ inputs.build-script }} ${{ inputs.group-key }} ${{ inputs.build-type }}
+        else
+          echo "cache hit do not prepare rtl"
+        fi
+      shell: bash
+