build crt0.s as part of build.rs

build.rs

@@ -0,0 +1,22 @@
+fn main() {
+    println!("cargo:rerun-if-changed=crt0.s");
+    println!("cargo:rerun-if-changed=interrupt_simple.s");
+
+    let out_file_name = "crt0.o";
+    let out_dir = std::env::var("OUT_DIR").expect("OUT_DIR environment variable must be specified");
+    let out_file_path = format!("{}/{}", out_dir, &out_file_name);
+
+    let out = std::process::Command::new("arm-none-eabi-as")
+        .arg("-mthumb-interwork")
+        .arg("-mthumb")
+        .args(&["-o", out_file_path.as_str()])
+        .arg("crt0.s")
+        .output()
+        .expect("failed to compile crt0.s");
+
+    if !out.status.success() {
+        panic!("{}", String::from_utf8_lossy(&out.stderr));
+    }
+
+    println!("cargo:rustc-link-search={}", out_dir);
+}