MFC r317928

Add necessary bits to get FreeBSD booting on the Unifi Security Gateway

Summary:
The Ubiquiti Unifi Security Gateway is virtually identical to the
EdgeRouter Lite, with a smaller PCB and apparently a different board identifier.
Simply adding the new board identifier alongside the ERL identifier, FreeBSD
boots successfully, and can access the needed peripherals.

Approved by: re(bdrewery)