The bootchooser's documentation states the 'namespace' for the state
storage backend is optional. This change makes it really optional to allow
a flat state variable set definition for the 'bootchooser' as well.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>