nullcons support for amd64/i386

[Ignatios Souvatzis <is%netbsd.org@localhost>]

Hi,

I was in urgent need for a console-less kernel, and found some
committed cobalt snippets (pseudo-device nullcons & friends) that
need more support for x86.

The diff (and the config file I use for it with amd64) are below.

I'd commit this, but it has been suggested to me that a generic
fall back to nullcons for all architectures would be preferable.

	-is