make firmware check fails on bridge_net target for arty

[mithro]

- make firmware check (arty bridge_net lm32 firmware)                                                                                                                                                      [0/9315]
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mkdir -p build/arty_bridge_net_lm32/
time python -u ./make.py --platform=arty --target=bridge_net --cpu-type=lm32 --iprange=192.168.100     --no-compile-gateware \
        2>&1 | tee -a build/arty_bridge_net_lm32//output.20181004-000927.log; (exit ${PIPESTATUS[0]})
Traceback (most recent call last):
  File "./make.py", line 164, in <module>
    main()
  File "./make.py", line 123, in main
    soc = get_soc(args, platform)
  File "./make.py", line 56, in get_soc
    exec("from targets.{}.{} import SoC".format(args.platform, args.target.lower(), args.target), globals())
  File "<string>", line 1, in <module>
ImportError: cannot import name 'SoC'