API: Add support for type aliases

Previously all types are compound. This adds support for aliases,
so one can do things like:

typedef u32 interface_index;

or
typedef u8 ip4_address[4];

Change-Id: I0455cad0123fc88acb491d2a3ea2725426bdb246
Signed-off-by: Ole Troan <ot@cisco.com>
Signed-off-by: Klement Sekera <ksekera@cisco.com>
diff --git a/src/vpp-api/python/vpp_papi/vpp_serializer.py b/src/vpp-api/python/vpp_papi/vpp_serializer.py
index bd0f738..a001cca 100644
--- a/src/vpp-api/python/vpp_papi/vpp_serializer.py
+++ b/src/vpp-api/python/vpp_papi/vpp_serializer.py
@@ -79,7 +79,7 @@
         self.packer = BaseTypes(field_type, num)
         self.size = self.packer.size
 
-    def pack(self, list, kwargs):
+    def pack(self, list, kwargs = None):
         """Packs a fixed length bytestring. Left-pads with zeros
         if input data is too short."""
         if not list:
@@ -277,6 +277,18 @@
         return self.tuple._make(r), maxsize
 
 
+def VPPTypeAlias(name, msgdef):
+    t = vpp_get_type(msgdef['type'])
+    if not t:
+        raise ValueError()
+    if 'length' in msgdef:
+        if msgdef['length'] == 0:
+            raise ValueError()
+        types[name] = FixedList(name, msgdef['type'], msgdef['length'])
+    else:
+        types[name] = t
+
+
 class VPPType(object):
     # Set everything up to be able to pack / unpack
     def __init__(self, name, msgdef):