Fix sonic-cfggen --hwsku|-k command (sonic-net#33)

Signed-off-by: Sangita Maity <sangitamaity0211@gmail.com>

src/sonic-config-engine/sonic-cfggen

@@ -37,7 +37,7 @@ from functools import partial
 from minigraph import minigraph_encoder
 from minigraph import parse_xml
 from minigraph import parse_device_desc_xml
-from portconfig import get_port_config, get_breakout_mode
+from portconfig import get_port_config, get_port_config_file_name, get_breakout_mode
 from sonic_device_util import get_machine_info
 from sonic_device_util import get_platform_info
 from sonic_device_util import get_system_mac
@@ -227,6 +227,8 @@ def main():
             'hwsku': hwsku
             }}}
         deep_update(data, hardware_data)
+        if args.port_config is None:
+            args.port_config = get_port_config_file_name(hwsku, platform)
         (ports, _) = get_port_config(hwsku, platform, args.port_config)
         if not ports:
             print('Failed to get port config', file=sys.stderr)