LU-12776 doc: update Changelog to reflect primary kernels

LU-12776 doc: update Changelog to reflect primary kernels

This patch updates the lustre/Changelog file to distinguish
between primary kernels that are built and tested during the
current release cycle from other kernels that are also known
to have built at some point.

Test-Parameters: trivial
Signed-off-by: Joseph Gmitter <jgmitter@whamcloud.com>
Change-Id: I7b6b5ce2dc813da91e2d9cd7a499168443e41f59
Reviewed-on: https://review.whamcloud.com/36214
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Peter Jones <pjones@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>