Align image ending
Currently we create the image based on the size of the rootfs + boot + freespace, but this means that the image isn't always aligned in its ending. Let's do something like Armbian does - see https://github.com/armbian/build/blob/3ab1bff4702b5f3eef334b60d8aeed76b9f84be0/lib/debootstrap.sh#L485