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/testsuite.at b/tests/testsuite.at
index 5029b9e..21fad1d 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -99,3 +99,16 @@
 AT_CHECK([$abs_top_builddir/tests/fr/fr_test], [], [expout], [experr])
 
 AT_CLEANUP
+
+AT_SETUP([loggingrb])
+AT_KEYWORDS([loggingrb])
+cat $abs_srcdir/loggingrb/logging_test.ok > expout
+cat $abs_srcdir/loggingrb/logging_test.err > experr
+AT_CHECK([$abs_top_builddir/tests/loggingrb/loggingrb_test], [], [expout], [experr])
+AT_CLEANUP
+
+AT_SETUP([strrb])
+AT_KEYWORDS([strrb])
+cat $abs_srcdir/strrb/strrb_test.ok > expout
+AT_CHECK([$abs_top_builddir/tests/strrb/strrb_test], [], [expout], [ignore])
+AT_CLEANUP