Portgroups are a bit of a complex item, unfortunately.

They hold a dual purpose of representing what is desired and what exists. The delineation between those states largely being what tooling is being loaded into Neutron in the form of a portgroup supporting ML2 drivers. If portgroups are pre-configured on a switch side, they can be represented in Ironic and the virtual port (VIF) binding information can be transmitted to Neutron with this information. If an ML2 driver is loaded that understands the portgroup configuration, then it can also configure the switch to represent this port. 

I can't tell if you're asking about pre-configured bonds or if you're asking about ML2 enabled bonds. I don't know if StarlingX ships with/uses any of the Neutron ML2 which support such functionality, and that may be a good question for the StarlingX community specifically. If you're just trying to express pre-configured portgroups, it looks like Cloud-init since 0.7.7 has apparently supported parsing and setting up the portgroup within the deployed operating system. That is, if it is present in that operating system image you deploy. Please note, caution should be taken with LACP[0] and various switch configuration tunables. They make network booting a bit complicated and each switch vendor has somewhat different behavior and configuration available to help navigate such situations.

-Julia

[0] https://docs.openstack.org/ironic/latest/admin/troubleshooting.html#why-does-x-issue-occur-when-i-am-using-lacp-bonding-with-ipxe