Adds the line `After=network-online.target` to the k3s systemd service
file. This applies the fix mentioned in
[this GH comment](https://github.com/rancher/k3s/issues/1626#issuecomment-642253812)
which I can confirm makes k3s networking survive reboot in Raspbian
Buster.

[It appears, in some docs I found](https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files)
that this is a recommended and usual way of specifying that we need the
target to be _completed_ before starting k3s. Using just the `Wants=`
directive doesn't work for this task, you have to add both directives
at once to do this. Quote:

> `Wants=`: This directive is similar to `Requires=`, but less strict.
> `Systemd` will attempt to start any units listed here when this unit
> is activated. If these units are not found or fail to start, the
> current unit will continue to function. This is the recommended way to
> configure most dependency relationships. **Again, this implies a
> parallel activation unless modified by other directives**

> [...]

> `After=`: The units listed in this directive will be started before
> starting the current unit. This does not imply a dependency
> relationship and **one must be established through the above
> directives if this is required.**

- _(Emphasis mine)_
Signed-off-by: Matthew Clive <arcticlight@arcticlight.me>
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>