gsm0808: Add CSFB indication IE to BSSMAP CLEAR COMMAND

When a call that was established in a CSFB context ends the CLEAR
COMMAND that is send from the BSC to the MSC should contain a CSFB
indication IE, which consists of just the IE byte itsslef. This
additional IE tells the BSC to include other CSFB related IEs into the
RR Release message.

Change-Id: Id8a75e1da2d5f520064666e4ee413d1c91da6ae3
Related: OS#3778
diff --git a/src/gsm/gsm0808.c b/src/gsm/gsm0808.c
index fce8e0a..2907255 100644
--- a/src/gsm/gsm0808.c
+++ b/src/gsm/gsm0808.c
@@ -195,6 +195,22 @@
 	return msg;
 }
 
+/*! Create BSSMAP Clear Command message.
+ *  \param[in] cause TS 08.08 cause value.
+ *  \param[in] csfb_ind indicate that the call was established in an CSFB context.
+ *  \returns callee-allocated msgb with BSSMAP Clear Command message. */
+struct msgb *gsm0808_create_clear_command2(uint8_t cause, bool csfb_ind)
+{
+	struct msgb *msg = gsm0808_create_clear_command(cause);
+	if (!msg)
+		return NULL;
+
+	if (csfb_ind)
+		msgb_v_put(msg, GSM0808_IE_CSFB_INDICATION);
+
+	return msg;
+}
+
 /*! Create BSSMAP Cipher Mode Command message
  *  \param[in] ei Mandatory Encryption Information
  *  \param[in] cipher_response_mode optional 1-byte Cipher Response Mode