add gsm0808_cell_id_from_cgi(), gsm0808_cell_id_to_cgi()

CGI to Cell ID: for example, for Paging, osmo-msc has a CGI for a subscriber
and needs to send out a Cell Identifier IE. Makes sense to add this conversion
here.

Cell ID to CGI: for a Layer 3 Complete, a subscriber sends the current cell in
the form of a Cell Identifier, which we store as a CGI, if necessary enriched
with the local PLMN.

Add enum with bitmask values to identify parts of a CGI, for the return value
of gsm0808_cell_id_to_cgi(). Can't use enum CELL_IDENT for that, because it
doesn't have a value for just a PLMN (and is not a bitmask).

Change-Id: Ib9af67b100c4583342a2103669732dab2e577b04
diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 2b48be7..53f145c 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -84,6 +84,9 @@
 bool gsm0808_cell_ids_match(const struct gsm0808_cell_id *id1, const struct gsm0808_cell_id *id2, bool exact_match);
 int gsm0808_cell_id_matches_list(const struct gsm0808_cell_id *id, const struct gsm0808_cell_id_list2 *list,
 				 unsigned int match_nr, bool exact_match);
+void gsm0808_cell_id_from_cgi(struct gsm0808_cell_id *cid, enum CELL_IDENT id_discr,
+			      const struct osmo_cell_global_id *cgi);
+int gsm0808_cell_id_to_cgi(struct osmo_cell_global_id *cgi, const struct gsm0808_cell_id *cid);
 
 uint8_t gsm0808_enc_cause(struct msgb *msg, uint16_t cause);
 uint8_t gsm0808_enc_aoip_trasp_addr(struct msgb *msg,
diff --git a/include/osmocom/gsm/gsm23003.h b/include/osmocom/gsm/gsm23003.h
index b34a677..cf622ce 100644
--- a/include/osmocom/gsm/gsm23003.h
+++ b/include/osmocom/gsm/gsm23003.h
@@ -30,6 +30,15 @@
 	uint16_t cell_identity;
 };
 
+/*! Bitmask of items contained in a struct osmo_cell_global_id.
+ * See also gsm0808_cell_id_to_cgi().
+ */
+enum osmo_cgi_part {
+	OSMO_CGI_PART_PLMN = 1,
+	OSMO_CGI_PART_LAC = 2,
+	OSMO_CGI_PART_CI = 4,
+};
+
 /* Actually defined in 3GPP TS 48.008 3.2.2.27 Cell Identifier List,
  * but conceptually belongs with the above structures. */
 struct osmo_lac_and_ci_id {