[mle] add enhanced keep-alive (#4325)

This commit introduces the first Thread 1.2 feature - enhanced keep
alive.

This commit also introduces the build config
OPENTHREAD_CONFIG_THREAD_VERSION to include/exclude Thread 1.2 code.

tests/scripts/thread-cert/message.py

@@ -354,6 +354,10 @@ class Message(object):
                           ipv6.UDPDatagram)
         return self.ipv6_packet.upper_layer_protocol.header.dst_port
 
+    def is_data_poll(self):
+        return self._type == MessageType.COMMAND and \
+            self._mac_header.command_type == mac802154.MacHeader.CommandIdentifier.DATA_REQUEST
+
     def __repr__(self):
         if (self.type == MessageType.DTLS and
                 self.dtls.content_type == dtls.ContentType.HANDSHAKE):
@@ -375,6 +379,14 @@ class MessagesSet(object):
     def commissioning_messages(self):
         return self._commissioning_messages
 
+    def next_data_poll(self):
+        while True:
+            message = self.next_message_of(MessageType.COMMAND, False)
+            if not message:
+                break
+            elif message.is_data_poll():
+                return message
+
     def next_coap_message(self, code, uri_path=None, assert_enabled=True):
         message = None