[pushkins]

Get yourself a masonry blade for your circular saw, that's the easiest way or you can by a backer board scoring knife for about $10 at Lowe's/Home Depot.
If you use the masonry blade in an old circular saw, cut a series of lines in the middle of your circle from one side to the other then you can simply score the circle perimeter with a sharp (not for long) Stanley knife and bend the pieces out.
Ideally I use a Rotozip with a masonry blade to do smaller holes.


NOTE wear a mask if your using the saw.