factory: populate /etc/locale.conf with systemd build-time setting

This adds /etc/locale.conf to the set of configuration files
populated by tmpfiles.d factory /etc handling.
In particular, the build-time locale configuration in systemd is
now wired to a /usr factory file, and installed to the system.
On boot, if other locale customization tools did not write
/etc/locale.conf on the system, the factory default file gets
copied to /etc by systemd-tmpfiles.
This is done in order to avoid skews between different system
components when no locale settings are configured. At that point,
systemd can safely act as the fallback owner of /etc/locale.conf.