default to ns for time unit -- synopsys dc whines

openfpga_flow/openfpga_shell_scripts/fix_device_route_chan_width_example_script.openfpga

@@ -59,7 +59,7 @@ write_verilog_testbench --file ./SRC --reference_benchmark_file_path ${REFERENCE
 
 # Write the SDC files for PnR backend
 #  - Turn on every options here
-write_pnr_sdc --time-unit ns --file ./SDC
+write_pnr_sdc --time_unit ns --flatten_names --file ./SDC
 
 # Write SDC to disable timing for configure ports
 write_sdc_disable_timing_configure_ports --file ./SDC/disable_configure_ports.sdc