They seem like a pair of ordinary advertising billboards, competing for the attention of drivers among the clutter of signage on busy Route 1. But a disagreement over who controls the billboards near Gillette Stadium is part of a dispute that has damaged the relationship between town government and its largest taxpayer, The Kraft Group, run by Patriots owner Robert Kraft. The Kraft Group insists the billboards are on its land. Foxborough officials say an easement gives the town control.