abis: Create a routine that can parse all SW Descriptions of a SW Config

Be able to parse the entire SW Config IE. Parse the SW Descruption
into a struct provided by the caller.
diff --git a/openbsc/tests/Makefile.am b/openbsc/tests/Makefile.am
index cede8e8..7ea4cff 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -1,4 +1,4 @@
-SUBDIRS = gsm0408 db channel mgcp gprs si
+SUBDIRS = gsm0408 db channel mgcp gprs si abis
 
 if BUILD_NAT
 SUBDIRS += bsc-nat