firstboot: add new component to query basic system settings on first boot, or when...

A new tool "systemd-firstboot" can be used either interactively on boot,
where it will query basic locale, timezone, hostname, root password
information and set it. Or it can be used non-interactively from the
command line when prepareing disk images for booting. When used
non-inertactively the tool can either copy settings from the host, or
take settings on the command line.

$ systemd-firstboot --root=/path/to/my/new/root --copy-locale --copy-root-password --hostname=waldi

The tool will be automatically invoked (interactively) now on first boot
if /etc is found unpopulated.

This also creates the infrastructure for generators to be notified via
an environment variable whether they are running on the first boot, or
not.