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/tests/Makefile.am b/openbsc/tests/Makefile.am
index 09298a3..ba5ca28 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -1,4 +1,4 @@
-SUBDIRS = gsm0408 db channel mgcp gprs abis gbproxy trau subscr mm_auth
+SUBDIRS = gsm0408 db channel mgcp gprs abis gbproxy trau subscr mm_auth xid
 
 if BUILD_NAT
 SUBDIRS += bsc-nat bsc-nat-trie