If passed, the path given to -reference_checkpoint is used to
`read_checkpoint -incremental` at the appropriate place in the
design flow for Vivado to do an incremental implementation

Also adds `report_incremental_reuse -hierarchical` reporting
at the various stages of the flow.

…cl.sh

trying to do it after link_design or after CL_synthesis leads to an error
saying

  ERROR: [Vivado 12-4846] Found memory core that needs to be (re)generated. Please run opt_design or
  implement_mig_cores prior to launching 'read_checkpoint -incremental'.