add BSSLAP coding for Location Services

BSSLAP: there are APDUs transferred in BSSMAP-LE Connection Oriented
Information messages on Lb between BSC and SMLC.
Add BSSLAP coding for these APDU messages:
- TA Layer3
- TA Request
- TA Response, possibly containing Location Estimate coded in GAD
- Reject
- Reset (for intra-BSS handover during TA Request)
- Abort (for inter-BSS handover)

Add encoding and decoding tests.

Change-Id: I6409c4bcac402dc7626a3afce9081c59cd715fe8
diff --git a/include/osmocom/gsm/bsslap.h b/include/osmocom/gsm/bsslap.h
new file mode 100644
index 0000000..b2174b3
--- /dev/null
+++ b/include/osmocom/gsm/bsslap.h
@@ -0,0 +1,57 @@
+/*! \addtogroup bsslap
+ *  @{
+ *  \file bsslap.h
+ * Message encoding and decoding for 3GPP TS 48.071 BSS LCS Assistance Protocol (BSSLAP).
+ */
+/*
+ * (C) 2020 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * Author: Neels Hofmeyr <neels@hofmeyr.de>
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ */
+#pragma once
+
+#include <osmocom/gsm/protocol/gsm_48_071.h>
+#include <osmocom/gsm/protocol/gsm_49_031.h>
+
+struct msgb;
+
+struct osmo_bsslap_err {
+	int rc;
+	enum bsslap_msgt msg_type;
+	enum bsslap_iei iei;
+	enum lcs_cause cause;
+	char *logmsg;
+};
+
+extern const struct value_string osmo_bsslap_msgt_names[];
+static inline const char *osmo_bsslap_msgt_name(enum bsslap_msgt val)
+{ return get_value_string(osmo_bsslap_msgt_names, val); }
+
+extern const struct value_string osmo_bsslap_iei_names[];
+static inline const char *osmo_bsslap_iei_name(enum bsslap_iei val)
+{ return get_value_string(osmo_bsslap_iei_names, val); }
+
+int osmo_bsslap_enc(struct msgb *msg, const struct bsslap_pdu *pdu);
+int osmo_bsslap_dec(struct bsslap_pdu *pdu,
+		    struct osmo_bsslap_err **err, void *err_ctx,
+		    const uint8_t *data, size_t len);
+
+/*! @} */