Use SPDX-License-Identifier instead of a verbose license text and
update external link.

Cc: James Guilford <james.guilford@intel.com>
Cc: Sean Gulley <sean.m.gulley@intel.com>
Cc: Chandramouli Narayanan <mouli@linux.intel.com>
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>