When configuring active-backup bonding devices in the kernel,
the module options provide a method for specifying a primary interface, e.g.:
options bonding mode=active-backup
options bonding primary=<link>
Without this option, the kernel will not switch over to the "better" <link>,
should it come up late or recover from a failure.
As of v219 systemd.netdev this feature is either unsupported, or undocumented.
Please use PrimarySlave= setting.