[Requirements] [neon] add a selective sign property to trigger on build if wanted

- the user will be able to trigger the sign if needed but t will not launch by default, saving some precious build time

Change-Id: I0aed0047e762524198c6728fe375e7b28ac86c08
Signed-off-by: Quentin Le Menez <quentin.lemenez@cea.fr>