boards: build/linker: need to expand search path

For relative imports to work in linker scripts we must include all of
the locations of the linker scripts in the linker search path. Since we
are already tracing through linker scripts we can do this fairly easily.

boards/build.rs

@@ -70,10 +70,15 @@ fn track_linker_script<P: AsRef<Path>>(path: P) {
 }
 fn track_linker_script_inner(linker_script: std::path::PathBuf, directory: std::path::PathBuf) {
     let path = std::path::absolute(directory.join(linker_script)).unwrap();
-    let parent_buf = path.parent().unwrap().to_path_buf();
+    let parent = path.parent().unwrap();
+    let parent_buf = parent.to_path_buf();
 
     assert!(path.is_file(), "expected path {path:?} to be a file");
 
+    // Make sure we include the path of the linker script in the search so we
+    // can use relative imports inside the linker script.
+    println!("cargo:rustc-link-arg=-L{}", parent.display());
+
     println!("cargo:rerun-if-changed={}", path.display());
 
     // Find all the `INCLUDE <relative path>` lines in the linker script.