build/util: prevent infinite loop if target/ not found

Previously, if you attempted to use certain functions in the build_util
crate...and your output directory happened to not include any parent
directories with the exact name "target"...

...the build script would burn 100% of a core forever, until you killed
it.

This doesn't affect Hubris-proper but _does_ affect out of tree builds.
The original code was arguably wrong anyway, so, this fixes that.

build/util/src/lib.rs

@@ -95,7 +95,9 @@ pub fn expose_target_board() {
         std::path::PathBuf::from(std::env::var("OUT_DIR").unwrap());
     loop {
         let done = out_dir.file_name() == Some(OsStr::new("target"));
-        out_dir.pop();
+        if !out_dir.pop() {
+            panic!("can't find target/ in OUT_DIR");
+        }
         if done {
             break;
         }