gsm0808: Add utils for AoIP Transport Layer Address

The planned support for true A over IP requires the encoding and
decoding of a so called "AoIP Transport Layer Address" element.

This commt adds parsing functionality and tests for the element
mentioned above, however, it is not yet actively used.

Change-Id: I57933b0a06a3f54ec2a41e6ecb6ced9fbbc89332
diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
new file mode 100644
index 0000000..5fc1c88
--- /dev/null
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -0,0 +1,30 @@
+/* (C) 2016 by Sysmocom s.f.m.c. GmbH
+ * All Rights Reserved
+ *
+ * Author: Philipp Maier
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+#pragma once
+
+#include <sys/socket.h>
+
+/* Encode AoIP transport address element */
+uint8_t gsm0808_enc_aoip_trasp_addr(struct msgb *msg,
+				    const struct sockaddr_storage *ss);
+
+/* Decode AoIP transport address element */
+int gsm0808_dec_aoip_trasp_addr(struct sockaddr_storage *ss,
+				const uint8_t *elem, uint8_t len);