Update changelog for 5.2.1

Change-Id: Id1df662afb2ec0e7eb7055bbc2c5c523a5bda125
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: If97b68b56071e241c659ddd231f28e433954d7be