build: emit assembler messages as warnings

build.rs

@@ -1,3 +1,4 @@
+use std::path::PathBuf;
 use std::{env, ffi::OsStr, io, io::Write, path::Path, process::Command};
 
 // We have to build stage 0 completely separately because it won't be part of
@@ -10,7 +11,7 @@ fn main() {
     let stage0_srcs = ["src/boot/stage0.s"];
     let stage0_objs = make.all(stage0_srcs);
     make.link(Some("config/stage0.ld"), stage0_objs, "stage0.elf");
-    make.objcopy("stage0.elf", "mbr.bin");
+    make.objcopy("stage0.elf", "stage0.bin");
 
     make.end();
 }
@@ -187,9 +188,14 @@ impl Make {
         eprintln!("  {cmd:?}");
 
         let output = cmd.output().expect("failed to execute assembler");
-        self.stdout_raw(output.stdout.as_ref());
+        let stderr = String::from_utf8(output.stderr).unwrap();
+        for line in stderr.lines() {
+            if !line.is_empty() {
+                println!("cargo:warning={line}");
+            }
+        }
+
         if !output.status.success() {
-            self.stdout_raw(output.stderr.as_ref());
             self.cmd_failed(&self.cmd_as, output.status.code());
         }
     }