Overlay dirs
============

1. The rootfs/*/ would be automatically installed into the rootfs.

2. The <OS>/ would be automatically installed into the related OS (including recovery, etc.).

3. Set RK_ROOTFS_EXTRA_OVERLAY_DIRS for custom rootfs overlay dirs.

4. Unset RK_ROOTFS_OVERLAY to disable rootfs overlays.

5. Unset RK_OVERLAY to disable all overlays.

If there's an install.sh under the overlay dir, it would be executed instead of
the default rsync installing.