ci(firmware): add drone config for firmware builds

.drone.yml

@@ -0,0 +1,17 @@
+---
+
+kind: pipeline
+type: docker
+name: default
+
+steps:
+  - name: build firmware for faderboard
+    image: git.luj0ga.de/luca/docker-embedded-rust
+    commands:
+      - >-
+        [ -z "$(git diff --name-only "$DRONE_COMMIT_BEFORE" "$DRONE_COMMIT_AFTER" | grep '^firmware/faderboard/')" ]
+        && (echo "nothing to do" >&2; exit 0)
+      - cargo build
+    when:
+      event:
+        - push