Copy all .link interface naming definitions to initramfs

Currently, only the default .link files from /lib/systemd/network are
copied to the initramfs, meaning, that additional (local) .link naming
definitions will be ignored when the system is still controlled by the
initramfs and interfaces might get wrong names.

Fix this by copying all .link files from /lib/systemd/network and
/etc/systemd/network into the initramfs, with the files from
/etc/systemd/network taking precedence over files with the same name
from /lib/systemd/network, as stated in the systemd.link manpage.

Closes: #793374