networking: document how to customize a NIC name

[dustymabe]

This is somewhat described in coreos/fedora-coreos-tracker#553. For now, because of what we decided, you'd need to do:

In all cases, apply the name to the real root:

storage:
  files:
    - path: /etc/systemd/network/25-infra.link
      mode: 0644
      contents:
        inline: |
          [Match]
          MACAddress=12:34:56:78:9a:bc
          [Link]
          Name=infra

In the initrd via kernel argument only if you need this customization in the initrd:

ifname=infra:12:34:56:78:9a:bc

See coreos/fedora-coreos-tracker#553 for why you might want this.

Acceptance Criteria:

• document how to set network interface via link file
• (optional) document how to set network interface using kernel arguments

[dustymabe]

@kelvinfan001 is working on this!