Allow using the serial number for defining the bus order

If present in the interface_map of the network proposal a serial_number can be
use additionally to the Product Name for defining a specific bus_order for a
specific node.
Merge pull request #137 from rhafer/pull-req-release-pebbles-master-ead8713cdc2ce28d97bab36901b28238e32bb08e

Allow using the serial number of a node as an additional matching criteria for the bus_order [1/3]