Add generic Mobile Identity encoder

Add generic function which allows caller to set Mobile Identity
explicitly. This allows to use IMEI or IMEISV for example. Make
gsm48_generate_mid_from_imsi() into wrapper around new function.

Change-Id: Id79be7abfff75ecd0d248bbeed93e605abeec9b3
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 53b6f6e..7a74718 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -246,6 +246,7 @@
 gsm48_encode_ssversion;
 gsm48_encode_useruser;
 gsm48_generate_lai;
+gsm48_generate_mid;
 gsm48_generate_mid_from_imsi;
 gsm48_generate_mid_from_tmsi;
 gsm48_mi_to_string;