OK, I can change to a function.

Actually I only test `kerneltype="airy"` ... `kerneltype="brickwall"` is the same thing:
http://docs.astropy.org/en/latest/_generated/astropy.nddata.convolution.make_kernel.make_kernel.html

I think allowing both "airy" and "brickwall" only breeds confusion and doesn't help anyone. I propose to remove "brickwall". @kevlavich @eteq Do you agree? I could make the change in this PR.