firstboot: Do not dereference symlinks

firstboot: Do not dereference symlinks

Let's always operate on paths without resolving the final component.
If the path is a symlink, it could point to a vendor default in /usr,
in which case we definitely do not want to modify the vendor defaults.
To avoid this from happening, we replace the symlink with our own file
instead of modifying the file the symlink points at.