add BSD build instructions

BSD.md

@@ -0,0 +1,60 @@
+# Building Rust embedded firmware on BSD
+
+Using the Rust stable compiler from the BSD ports is mostly a matter of removing the arbitrary limitations that have been built into the Rust toolchain.
+
+Get rid of the "nightly toolchain" check in cargo-xbuild:
+```
+diff --git a/src/lib.rs b/src/lib.rs
+index 204ad8f..9c61e39 100644
+--- a/src/lib.rs
++++ b/src/lib.rs
+@@ -157,28 +157,11 @@ pub fn build(args: Args, command_name: &str, crate_config: Option<Config>) -> Re
+         })
+     })?;
+
+-    // We can't build sysroot with stable or beta due to unstable features
+     let sysroot = rustc::sysroot(verbose)?;
+-    let src = match meta.channel {
+-        Channel::Dev => rustc::Src::from_env().ok_or(anyhow!(
++    let src = rustc::Src::from_env().ok_or(anyhow!(
+             "The XARGO_RUST_SRC env variable must be set and point to the \
+-             Rust source directory when working with the 'dev' channel",
+-        ))?,
+-        Channel::Nightly => {
+-            if let Some(src) = rustc::Src::from_env() {
+-                src
+-            } else {
+-                sysroot.src()?
+-            }
+-        }
+-        Channel::Stable | Channel::Beta => {
+-            bail!(
+-                "The sysroot can't be built for the {:?} channel. \
+-                 Switch to nightly.",
+-                meta.channel
+-            );
+-        }
+-    };
++             Rust source directory",
++        ))?;
+
+     let cmode = if let Some(triple) = args.target() {
+         if triple == meta.host {
+```
+
+Run ``cargo install --path .`` to install the modified cargo-xbuild
+
+Get a copy of the Rust sources that corresponds to the installed compiler binary, and set up the environment accordingly:
+```
+> rustc --version
+rustc 1.72.1 (d5c2e9c34 2023-09-13) (built from a source tarball)
+> git clone --depth=1 --recurse-submodules --shallow-submodules https://github.com/rust-lang/rust --branch 1.72.1
+> export XARGO_RUST_SRC=`pwd`/rust/library
+```
+
+Disable other arbitrary checks in the Rust compiler:
+```
+> export RUSTC_BOOTSTRAP=1
+```
+
+And you can now simply run ``cargo xbuild`` to build the firmware.

README.md

@@ -23,6 +23,8 @@ The resulting ELF file will be located under `target/thumbv7em-none-eabihf/relea
 
 Alternatively, you can install the Rust toolchain without Nix using rustup; see the Rust manifest file pulled in `flake.nix` to determine which Rust version to use.
 
+For building on FreeBSD or OpenBSD, see BSD.md.
+
 ## Debugging
 
 Connect SWDIO/SWCLK/RST/GND to a programmer such as ST-Link v2.1. Run OpenOCD: