examples/er-rest-example: Add battery resource.

Adds battery resource to the example server.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add battery sensor.

Add battery voltage sensor to the platform.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>