Add a sysfs feature label

The new label `io.balena.features.sysfs` will mount the host /sys into
the container at /sys.

Relates to issue: #743

Change-type: minor
Signed-off-by: Cameron Diver <cameron@balena.io>

src/compose/utils.ts

@@ -370,6 +370,7 @@ export function addFeaturesFromLabels(
 				`http://${host}:${options.listenPort}`,
 			);
 		},
+		'io.balena.features.sysfs': () => service.config.volumes.push('/sys:/sys'),
 	};
 
 	_.each(features, (fn, label) => {