udev: link-config - sanity check the ifname and mac address
udev: link-config - move naming policy from udev rules

This introduces a new key NamePolicy, which takes an ordered list of naming
policies. The first successful one is applide. If all fail the value of Name
(if any) is used.

The possible policies are 'onboard', 'slot', 'path' and 'mac'.

This patch introduces a default link file, which replaces the equivalent udev
rule.