systemd: actively disable Predictable Network Interface Names if !pni-names

systemd: actively disable Predictable Network Interface Names if !pni-names

Currently pni-names PACKAGECONFIG option adds "mac" names policy if
selected, but nothing otherwise. In systemd Predictable Network Interface
Names are applied by default which leads to inconsistend behaviour.

One could think that pni-names in DISTRO_FEATURES would enable or disable
Predictable Network Interface Names, but currently if not present it's
only actively disabled for QEMU via kernel command line
(commit 9e9c33d51e40
("qemuboot/runqemu: Fix 6.2 and later kernel network device naming")).
It has no effect on the real HW. If the option is present, it merely adds
"mac" into default NamePolicy, which may or may not have observable
effects.

Make pni-names semantics more consistent by actively suppressing
Predictable Network Interface Names if the feature is not present.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>