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/include/osmocom/Makefile.am b/include/osmocom/Makefile.am
index 246cb34..846fcc3 100644
--- a/include/osmocom/Makefile.am
+++ b/include/osmocom/Makefile.am
@@ -1,2 +1,2 @@
-SUBDIRS = hnbap ranap rua iuh
+SUBDIRS = hnbap ranap rua sabp iuh