Added a ring buffer log target to store the last N log messages.

The log target can be used via log alarms and show alarms.
Why? This feature was proposed/requested at
http://openbsc.osmocom.org/trac/wiki/Tasks/ErrorLogTarget
All messages use the same amount of space, prioritizing simplicity.
diff --git a/tests/strrb/strrb_test.ok b/tests/strrb/strrb_test.ok
new file mode 100644
index 0000000..9ac5124
--- /dev/null
+++ b/tests/strrb/strrb_test.ok
@@ -0,0 +1 @@
+All tests passed