separateDebugInfo: fix cleanup, refactor a little

- cleanup with --ignore-fail-on-non-empty
- extract destDir and destFile variables
- unnest all the subshells

pkgs/build-support/setup-hooks/separate-debug-info.sh

@@ -33,21 +33,25 @@ _separateDebugInfo() {
 
         # Extract the debug info.
         echo "separating debug info from $i (build ID $id)"
-        mkdir -p "$dst/${id:0:2}"
+
+        destDir=$dst/${id:0:2}
+        destFile=$dst/${id:0:2}/${id:2}.debug
+
+        mkdir -p "$destDir"
+
+        if [ -f "$destFile" ]; then
+            echo "separate-debug-info: warning: multiple files with build id $id found, overwriting"
+        fi
 
         # This may fail, e.g. if the binary is for a different
         # architecture than we're building for.  (This happens with
         # firmware blobs in QEMU.)
-        (
-            if [ -f "$dst/${id:0:2}/${id:2}.debug" ]
-            then
-                echo "separate-debug-info: warning: multiple files with build id $id found, overwriting"
-            fi
-            (
-                $OBJCOPY --only-keep-debug "$i" "$dst/${id:0:2}/${id:2}.debug" &&
-                # Also a create a symlink <original-name>.debug.
-                ln -sfn ".build-id/${id:0:2}/${id:2}.debug" "$dst/../$(basename "$i")"
-            )
-        ) || rmdir -p "$dst/${id:0:2}"
+        if $OBJCOPY --only-keep-debug "$i" "$destFile"; then
+            # If we succeeded, also a create a symlink <original-name>.debug.
+            ln -sfn ".build-id/${id:0:2}/${id:2}.debug" "$dst/../$(basename "$i")"
+        else
+            # If we failed, try to clean up unnecessary directories
+            rmdir -p "$dst/${id:0:2}" --ignore-fail-on-non-empty
+        fi
     done < <(find "$prefix" -type f -print0 | sort -z)
 }