gsm0808: Add gsm0808_create_common_id()

This function encodes a GSM 08.08 / 48.008 "Common ID" message.

Change-Id: I353adc1aa72377f7d4b3336d2ff47791fb73d62c
Related: OS#2969
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 2d65372..8aa2367 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -13,3 +13,4 @@
 gsm		new API			new osmo_bts_unset_feature()
 gb		API/ABI change		deprecate gprs_nsvc_crate(); export gprs_nsvc_create2()
 gsm		API/ABI change		add new member to lapd_datalink
+gsm		new API			new gsm0808_create_common_id()