GPRS: Introduce a GPRS Gb Proxy

The ida of the Gb proxy is to aggregate Gb links with a number of BSS
and then present all the BSSGP-VC's together inside one NS-VC to the
actual SGSN.

The code is not yet expected to be complete.
diff --git a/openbsc/src/gprs_bssgp.c b/openbsc/src/gprs_bssgp.c
index 650d7d4..a2181b1 100644
--- a/openbsc/src/gprs_bssgp.c
+++ b/openbsc/src/gprs_bssgp.c
@@ -74,11 +74,6 @@
 	return "undefined";
 }
 
-static inline int bssgp_tlv_parse(struct tlv_parsed *tp, u_int8_t *buf, int len)
-{
-	return tlv_parse(tp, &tvlv_att_def, buf, len, 0, 0);
-}
-
 static inline struct msgb *bssgp_msgb_alloc(void)
 {
 	return msgb_alloc_headroom(4096, 128, "BSSGP");
@@ -120,7 +115,7 @@
 }
 
 /* Chapter 10.4.14: Status */
-static int bssgp_tx_status(u_int8_t cause, u_int16_t *bvci, struct msgb *orig_msg)
+int bssgp_tx_status(u_int8_t cause, u_int16_t *bvci, struct msgb *orig_msg)
 {
 	struct msgb *msg = bssgp_msgb_alloc();
 	struct bssgp_normal_hdr *bgph =