nixos: make several kernel common-config options optional

Realistically, common-config is useful, but there are a lot of things in
there that are non-optionally specified that aren't always useful. For
example, when deploying grsecurity, I don't want the bluetooth,
wireless, or input joystick/extra filesystem stack (XFS, etc), nor the
staging drivers tree.

The problem is that if you specify this in your own kernel config in the
grsecurity module, by saying 'BT n' to turn off bluetooth,
common-config turns on 'BT_HCIUART_BCSP y', which then becomes unused
and errors out.

This is really just an arbitrary picking at the moment, but it should be
OK.

Signed-off-by: Austin Seipp <aseipp@pobox.com>