add eth platform and --interfaces command line option

When using the eth platform you can give oft multiple "-i ofport@interface"
options to specify which ethernet interfaces to use for the dataplane, instead
of needing to write your own platform file.
diff --git a/platforms/eth.py b/platforms/eth.py
new file mode 100644
index 0000000..5628092
--- /dev/null
+++ b/platforms/eth.py
@@ -0,0 +1,28 @@
+"""
+Eth platform
+
+This platform uses the --interface command line option to choose the ethernet interfaces.
+"""
+
+def platform_config_update(config):
+    """
+    Update configuration for the local platform
+
+    @param config The configuration dictionary to use/update
+    """
+
+    port_map = {}
+
+    for (ofport, interface) in config["interfaces"]:
+        port_map[ofport] = interface
+
+    # Default to a veth configuration compatible with the reference switch
+    if not port_map:
+        port_map = {
+            1: 'veth1',
+            2: 'veth3',
+            3: 'veth5',
+            4: 'veth7',
+        }
+
+    config['port_map'] = port_map