Use Bdale's hack for clearing copper away from cut-outs

Use Bdale's hack for clearing copper away from cut-outs

This places an explicit copper trace along the edge of the cut-out
area, with clearline set so that the fill on the rest of the board is
kept away from the edge, leaving a clear space for the router to pass
through.

Signed-off-by: Keith Packard <keithp@keithp.com>