Update changelog for 5.2.1

Change-Id: Ic9c96f04ee00ac04dc3f3c1abdb09deb129eb5a4
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: I3c7e65df05c1aa3eec86eaf81a7fa1d263f7a22b
Merge "Merge remote-tracking branch 'origin/release' into stable" into refs/staging/stable