kernel/configs: Replace GPL boilerplate code with SPDX identifier

The FSF does not reside in "675 Mass Ave, Cambridge" anymore...
let's replace the old GPL boilerplate code with a proper SPDX
identifier instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>