Adding LLC-XID encoder / decoder and unit test

The lle-xid encoder/decoder is needed to encode and decode llc
xid parameter messages. We need this to exchange sndcp-parameters
(SNDCP-XID) and also simple parameters such as encryption IOVs

Change-Id: Ia06e4cb08bf9b48c2a4682606d1b1a91d19a9d37
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 6dc7e16..8cbdd91 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -28,7 +28,7 @@
 			sgsn_ctrl.c sgsn_auth.c gprs_subscriber.c \
 			gprs_utils.c gprs_gsup_client.c \
 			sgsn_cdr.c sgsn_ares.c \
-			oap.c oap_messages.c
+			oap.c oap_messages.c gprs_llc_xid.c
 osmo_sgsn_LDADD = 	\
 			$(top_builddir)/src/libcommon/libcommon.a \
 			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS) $(LIBCARES_LIBS) \