gsm0808_utils: Introduce gsm0808_msgb_put_cell_id_u()

This function is doing the bulk work of encoding a given Cell
ID List item.  gsm0808_enc_cell_id_list2() is modified to be a
wrapper / loop around the new function.

The purpose of this is to expose Cell ID List Entry encoding
so that the upcoming CBSP protocol encoder can re-use this code.

Related: OS#3537
Change-Id: I6cc567798e20365e6587e6b2988e834306d8c80c
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index adfa096..840bac9 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -242,6 +242,7 @@
 gsm0808_lcls_status_names;
 gsm0808_enc_lcls;
 gsm0808_dec_lcls;
+gsm0808_msgb_put_cell_id_u;
 
 gsm29118_msgb_alloc;
 gsm29118_create_alert_req;