Allow using the serial number for defining the bus order

This introduces a new optional attribute "serial_number" in the interface_maps
that can be use as additional matching criteria for defining the interface
order for a certain node. (This is helpful e.g. if two nodes with the same
Product Name need to have different interface_map defined)
Handle empty network definitions in recipe

This just makes the code more solid.
Merge pull request #193 from vuntz/avoid-crash

Handle empty network definitions in recipe
Merge pull request #192 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 [3/3]