mgmt/mcumgr/grp/img: No MCUboot flags setting in DirectXIP

It is no longer possible to select image for test or confirm
when MCUboot has been configured for DirectXIP.
MCUboot flags, in image trailer, are now ignored in DirectXIP
mode allowing to use entire slot for image upload.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>