System: Add support for links from externally-controlled boards

In order to support testing with boards that are not yet controlled via the online software framework, we need to add the concept of externally-controlled boards, and links that are connected to them

Tentative idea for config file format:

  external_boards:
    <board_name>:
      crate: <crate_id>
      slot: <slot_number>
      outputs:
         <processor_id>: <port_indices>