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/src/.gitignore b/src/.gitignore
index 55bca01..0b4676f 100644
--- a/src/.gitignore
+++ b/src/.gitignore
@@ -7,4 +7,7 @@
 ranap_decoder.c
 ranap_encoder.c
 
+sabp_decoder.c
+sabp_encoder.c
+
 hnbgw