github: Add workflow to test mynewt builds

This reproduces the CI test build for Mynewt that is currently run by
Travis.

Signed-off-by: David Brown <david.brown@linaro.org>

.github/workflows/mynewt.yaml

@@ -0,0 +1,28 @@
+# For development, trigger this on any push.
+on: [push, pull_request]
+
+# Eventually, we'll want to limit this to pull requests.
+# on: ???
+
+jobs:
+  environment:
+    name: Mynewt build
+    runs-on: ubuntu-latest
+    env:
+      MULTI_FEATURES: ${{ matrix.features }}
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: recursive
+    - name: Print the environment
+      run: |
+        uname -a
+        lscpu
+        free
+        pwd
+    - name: Mynewt install
+      run: |
+        ./ci/mynewt_install.sh
+    - name: Mynewt run
+      run: |
+        ./ci/mynewt_run.sh