When configuring active-backup bonding devices in the kernel,
the module options provide a method for specifying a primary interface, e.g.:

$ cat/etc/modprobe.d/bonding.conf
options bonding mode=active-backup
options bonding primary=<link>

Ref. linux/Documentation/networking/bonding.txt

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.