blocklist: Add vendor import instructions
Add vendor import instructions for blocklist.
It includes a "freebsd-changes.sh" script that takes care of adapting
paths and functions into FreeBSD.
Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49510
(cherry picked from commit 715cc257137099e022c7eecea4cd6995bb7604a3)