extract_files: Add support for paths without system/

Sometime blob directories are contained in dirs not named system
(ex. extracting blobs from another vendor dir) so check if the file
exists without system/ before appending "system/" to filenames.

Change-Id: I4ba946e178260b4f4804b1ac4422645e3085c665

build/tools/extract_utils.sh

@@ -873,6 +873,8 @@ function extract() {
             TARGET="$FROM"
             OUTPUT_DIR="$OUTPUT_DIR/rootfs"
             TMP_DIR="$TMP_DIR/rootfs"
+        elif [ -f "$SRC/$FILE" ] && [ "$SRC" != "adb" ]; then
+            TARGET="$FROM"
         else
             TARGET="system/$FROM"
             FILE="system/$FILE"