Add a GSM RLP decoder and encoder

This code implements a decoder and encoder for the RLP (Radio Link
Protocol) as used in the bearer channel of GSM CSD (Circuit Switched
Data).

Change-Id: I2d9bd8eb4f0cd0f72c436996767b199429596917
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 6bb3b4b..db2dbcb 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -864,5 +864,12 @@
 osmo_csd_3k6_encode_frame;
 osmo_csd_ubit_dump;
 
+osmo_rlp_decode;
+osmo_rlp_encode;
+osmo_rlp_fcs_compute;
+osmo_rlp_ftype_s_vals;
+osmo_rlp_ftype_u_vals;
+osmo_rlp_ftype_vals;
+
 local: *;
 };