initramfs-framework: overlayroot: Don't assume rootfs location

initramfs-framework: overlayroot: Don't assume rootfs location

Instead of assuming that the old rootfs is located at /rootfs, use the
ROOTFS_DIR variable.

This makes it possible to change rootfs location between the rootfs
module and the overlayroot module.

Signed-off-by: Marcus Flyckt <marcus.flyckt@kvaser.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>