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/.gitignore b/.gitignore
index e1430f9..f662e2a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -34,6 +34,7 @@
 src/osmo-hnbgw
 tags
 libosmo-ranap.pc
+libosmo-sabp.pc
 m4
 gen_hnbap.stamp
 gen_rua.stamp
@@ -41,3 +42,4 @@
 include/osmocom/hnbap/hnbap_ies_defs.h
 include/osmocom/rua/rua_ies_defs.h
 include/osmocom/ranap/ranap_ies_defs.h
+include/osmocom/sabp/sabp_ies_defs.h