systemd: add patch to consider systemd.hostname= a static hostname

[hnez]

We use the systemd.hostname= kernel commandline parameter to set a unique hostname from factory data via barebox.

The current semantics result in somewhat unexpected behaviour when doing so. Clean up the semantics of the systemd.hostname=` kernel parameter to make it more useful and predictable.

The previous semantics were:

• systemd.hostname= takes precedence over /etc/hostname during boot, even though it is technically considered a transient hostname.
• In systemd-hostnamed however /etc/hostname takes precedence over systemd.hostname= as it is considered transient again and the normal priorities (static over transient over default) are followed. This means if a new transient hostname is set, the hostname of the system will be sethostname()'d to the one in /etc/hostname (if present) or the new transient hostname.
• If /etc/hostname does not exist, systemd-hostnamed does not report a static hostname via dbus to consumers like the NetworkManager dhcp client. In the case of NetworkManager this results in no hostname being set in DHCP requests.

The new semantics are:

• systemd.hostname= is considered a static hostname during boot and in systemd-hostnamed and as such takes precedence over transient or default hostnames.
• systemd.hostname= takes precedence over /etc/hostname.

[hnez]

Just to add some context on this PR and why it is not merged yet.
The patch was also sent upstream systemd/systemd#25158 and it does not look too likely that it will be merged.
We will likely put on our thinking cap again and think of other nice approaches how to set a static unique hostname on systems with read only root fs, like bind-mounting a config to /etc/hostname.