Add cross configuration (#69)

This allows running unit tests on a non-linux platform via the `cross
test` command.

Author: astapleton

.vscode/settings.json

@@ -1,8 +1,9 @@
 {
     "rust-analyzer.cargo.features": [
         "rt",
-        "stm32h503"
+        "stm32h503",
+        "async",
     ],
     "rust-analyzer.check.allTargets": false,
     "rust-analyzer.check.targets": "thumbv8m.main-none-eabihf",
-}
+}

Cross.toml

@@ -0,0 +1,10 @@
+# Cross doesn't seem to look for package specific configurations when invoked in a workspace so
+# we put the default target in Cross.toml
+
+# Allow usage of locally built version of the cross docker image that matches the host architecture
+# e.g. Building a linux/arm64 image for an AARCH64 host allows native execution of the image.
+# Specifying the toolchain for a linux/arm64 image means the correct toolchain is used on that
+# platform (it defaults to x86_64).
+[target.aarch64-unknown-linux-gnu.image]
+name = "ghcr.io/cross-rs/aarch64-unknown-linux-gnu:main"
+toolchain = ["linux/arm64=aarch64-unknown-linux-gnu"]

README.md

@@ -72,6 +72,20 @@ a number of fantastic resources available to help.
 - [Rust Embedded FAQ](https://docs.rust-embedded.org/faq.html)
 - [rust-embedded/awesome-embedded-rust](https://github.com/rust-embedded/awesome-embedded-rust)
 
+## Running unit tests
+
+If you're on a linux host, you can simply run unit tests with:
+
+    cargo test --target x86_64-unknown-linux-gnu --tests --features stm32h503
+
+If you're not, you'll need to install [`cross`](https://github.com/cross-rs/cross), which is a cross compilation tool. See the `cross` instructions for details of installation and use. Once that is installed, using `cross test` will run the unit tests within a docker image.
+
+Running tests with `cross`:
+
+    cross test --target x86_64-unknown-linux-gnu --tests --features stm32h503
+
+`cross` will automatically download the relevant docker image and install the correct rust toolchain.
+
 ## Changelog
 
 See [CHANGELOG.md](CHANGELOG.md). Note: this will be populated once the first crates have been