With PER_DEVICE_ROOTFS, rebuilding kernel is needed to embed the cpio
image in the kernel image. With ROOTFS_INITRAMFS_SEPARATE, the cpio
image is external hence we can reuse the same kernel image without
rebuilding it.
Link: https://github.com/openwrt/openwrt/pull/12959
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>