Add SW Description (de)marshalling

* data structure representing 3GPP TS 52.021 ยง9.4.62 SW Description
* function to serialize it into msgb
* function to deserialize it from buffer
* functions to extract/estimate buffer size for SW Description
* test harness (partially taken from OpenBSC)

There are several similar functions to deal with SW Description in
OpenBSC, there's also need to use similar functionality in
OsmoBTS. Hence it's better to put the code into common library with
proper tests and documentation.

Change-Id: Ib63b6b5e83b8914864fc7edd789f8958cdc993cd
Related: OS#1614
diff --git a/.gitignore b/.gitignore
index ecbcedd..4c6a78f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -60,6 +60,7 @@
 tests/testsuite
 tests/testsuite.dir/
 tests/testsuite.log
+tests/abis/abis_test
 tests/ctrl/ctrl_test
 tests/utils/utils_test
 tests/stats/stats_test