fix #456 tell kernel hostname

[CWempe]

make defined hostname available via hostname command during installation

[diederikdehaas]

Why isn't this called with chroot so it gets set in the target system?
It now looks like it's setting the hostname in the installer, which then in a way get propagated into the chroot (like magic as the dns resolver somehow worked before the introduction of the resolvconf package)

[CWempe]

I don't know.
I just had an issue (#456) and @Mausy5043 told me how to solve it. 😄

[kpfleming]

The hostname command is injecting information into the kernel, not the filesystem. It won't matter whether it is run in the installer-root or the installed-root.