SNDCP: add SNDCP-XID encoder/decoder and unit test

The SNDCP-XID (or layer-3 xid) is used to exchange layer-3 parameters
such as compression. The encoder encodes a bytestream that is then
sent as regular XID field from LLC.

We will need the SNDCP-XID to negotiate the parameters for our
upcomming GPRS data and header compression features

Change-Id: If2d63fe2550864cafef3156b1dc0629037c49c1e
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index f63d61f..9dc2f8d 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -231,6 +231,7 @@
     tests/gtphub/Makefile
     tests/mm_auth/Makefile
     tests/xid/Makefile
+    tests/sndcp_xid/Makefile
     doc/Makefile
     doc/examples/Makefile
     Makefile)