gsm0808: add function to convert amr gsm0408 setings to gsm0808

Add a function to convert struct gsm48_multi_rate_conf, which holds the
codec settings for AMR, to S0-S15 bit representation as defined in
3GPP TS 48.008 3.2.2.49

This resurrects change-id I4e656731b16621736c7a2f4e64d9ce63b1064e98
which was reverted in I9e0d405e303ed86d23703ca6362e958dddb2f861
due to gsm0808_test failing.

The test failure is fixed by properly clearing the struct
gsm48_multirate_cfg prior to running tests (add memset(0)).

Change-Id: Ia782e21c206c15e840226d79b4209d13658ee916
Related: OS#3548
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 0c40c88..3d08232 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -205,6 +205,7 @@
 gsm0808_cell_id_u_name;
 gsm0808_chan_type_to_speech_codec;
 gsm0808_speech_codec_from_chan_type;
+gsm0808_sc_cfg_from_gsm48_mr_cfg;
 gsm0808_speech_codec_type_names;
 gsm0808_permitted_speech_names;
 gsm0808_chosen_enc_alg_names;