sabp: Generate C/H files for SABP; create libosmo-sabp

This uses the (modified) Osmocom asn1c on the (modified) SABP ASN.1
syntax to generate C code + header files for SABP parsing/encoding.

It also adds some helper code for message encoding and decoding as well
as a new libosmo-sabp shared library which can be used by programs to
implement SABP related functionality.

Change-Id: Ib9580d1af96354398da4c9f97b28a0e23d56e275
diff --git a/configure.ac b/configure.ac
index 4293635..6328975 100644
--- a/configure.ac
+++ b/configure.ac
@@ -142,10 +142,12 @@
 
 AC_OUTPUT(
 	libosmo-ranap.pc
+	libosmo-sabp.pc
 	src/Makefile
 	src/hnbap/Makefile
 	src/ranap/Makefile
 	src/rua/Makefile
+	src/sabp/Makefile
 	src/tests/Makefile
 	Makefile
 	include/Makefile
@@ -153,6 +155,7 @@
 	include/osmocom/hnbap/Makefile
 	include/osmocom/ranap/Makefile
 	include/osmocom/rua/Makefile
+	include/osmocom/sabp/Makefile
 	include/osmocom/iuh/Makefile
 	doc/Makefile
 	doc/examples/Makefile