build: add directory-oriented make workflows

firmware/README.md

@@ -14,9 +14,15 @@ firmware/
 └── tests/      firmware integration-test fixtures
 ```
 
-Run Cargo commands from this directory:
+This workspace owns its Makefile. From this directory, list its commands with:
 
 ```sh
-cd firmware
-cargo test --workspace --locked --offline
+make help
 ```
+
+From the repository root, the firmware path is a direct Cargo proxy. For
+example, `make firmware -- test --workspace --locked --offline` runs the host
+test suite. Build and package every supported board with
+`make firmware/boards`, or select one board by its directory path.
+Run `make firmware clippy` for Rust lints and `make firmware docs` for API
+documentation with warnings denied.