Follow symlinks when finding link files to copy into initramfs

If the /{etc,lib}/systemd/network directory itself is a symlink, the find command will not actually find any of the files in the dir it links to. Use the find -L param to follow symlinks.

Note that the -L does not need to be provided to the cp command, as when using only the -p parameter symlinks are followed by default. Also, the [ -d ] test follows symlinks by default, and does not need changing.