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/abis/abis_test.ok b/openbsc/tests/abis/abis_test.ok
new file mode 100644
index 0000000..6401988
--- /dev/null
+++ b/openbsc/tests/abis/abis_test.ok
@@ -0,0 +1,9 @@
+Start: 0 len: 13
+file_id:  01 02 03
+file_ver: 03 04 05
+Start: 13 len: 26
+file_id:  01 02 03
+file_ver: 03 04 05
+Start: 26 len: 13
+file_id:  09 07 05
+file_ver: 06 07 08