[codec] add decoding for allowlists that are passed back from the NCP (…

…#125)

Add unpacking for the various types of responses that come back from
the MAC_ALLOWLIST property. Note that there are three different
formats, so we try them from the most complex/involved to the least
complex/involved.

1. Try to decode an array of structures composed of an EUI64 and an
   unsigned 8-bit integer (from querying the value)
2. If that fails, try to decode a single entry composed of an EUI64
   and an unsigned 8-bit integer (from inserting a value)
3. If that fails, try to decode a single entry composed of only an
   EUI64 (from deleting an entry)
4. If all of these fail, return None

spinel/codec.py

@@ -21,6 +21,7 @@
 import binascii
 import time
 import logging
+import struct
 import threading
 import traceback
 import queue
@@ -502,7 +503,13 @@ def MAC_RAW_STREAM_ENABLED(self, _, payload):
         return self.parse_b(payload)
 
     def MAC_ALLOWLIST(self, _, payload):
-        pass
+        formats = ["A(t(EC))", "EC", "E"]
+        for format in formats:
+            try:
+                return self.parse_fields(payload, format)
+            except struct.error:
+                pass
+        return None
 
     def MAC_ALLOWLIST_ENABLED(self, _, payload):
         return self.parse_b(payload)