network-generator: rewrite unit if it already exists and its content changed

network-generator: rewrite unit if it already exists and its content changed

When the `systemd-network-generator` is included in the initrd and runs from
there first, the next times it runs after switching to real root it
thinks there is a duplicate entry on the kernel command line.

This patch rewrites the unit file if the content has changed, instead of
displaying an error message.