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/loggingrb/logging_test.err b/tests/loggingrb/logging_test.err
new file mode 100644
index 0000000..b59d2e8
--- /dev/null
+++ b/tests/loggingrb/logging_test.err
@@ -0,0 +1,3 @@
+You should see this
+You should see this
+
\ No newline at end of file
diff --git a/tests/loggingrb/logging_test.ok b/tests/loggingrb/logging_test.ok
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tests/loggingrb/logging_test.ok
diff --git a/tests/loggingrb/loggingrb_test.c b/tests/loggingrb/loggingrb_test.c
new file mode 100644
index 0000000..1ab5212
--- /dev/null
+++ b/tests/loggingrb/loggingrb_test.c
@@ -0,0 +1,83 @@
+/* simple test for the debug interface */
+/*
+ * (C) 2008, 2009 by Holger Hans Peter Freyther <zecke@selfish.org>
+ * (C) 2012-2013 by Katerina Barone-Adesi <kat.obsc@gmail.com>
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+#include <assert.h>
+
+#include <osmocom/core/logging.h>
+#include <osmocom/core/utils.h>
+#include <osmocom/core/ringb.h>
+#include <osmocom/vty/logging_rbvty.h>
+
+enum {
+	DRLL,
+	DCC,
+	DMM,
+};
+
+static const struct log_info_cat default_categories[] = {
+	[DRLL] = {
+		  .name = "DRLL",
+		  .description = "A-bis Radio Link Layer (RLL)",
+		  .color = "\033[1;31m",
+		  .enabled = 1, .loglevel = LOGL_NOTICE,
+		  },
+	[DCC] = {
+		 .name = "DCC",
+		 .description = "Layer3 Call Control (CC)",
+		 .color = "\033[1;32m",
+		 .enabled = 1, .loglevel = LOGL_NOTICE,
+		 },
+	[DMM] = {
+		 .name = NULL,
+		 .description = "Layer3 Mobility Management (MM)",
+		 .color = "\033[1;33m",
+		 .enabled = 1, .loglevel = LOGL_NOTICE,
+		 },
+};
+
+const struct log_info log_info = {
+	.cat = default_categories,
+	.num_cat = ARRAY_SIZE(default_categories),
+};
+
+int main(int argc, char **argv)
+{
+	struct log_target *ringbuf_target;
+
+	log_init(&log_info, NULL);
+	ringbuf_target = log_target_create_rbvty(NULL, 0x1000);
+	log_add_target(ringbuf_target);
+	log_set_all_filter(ringbuf_target, 1);
+	log_set_print_filename(ringbuf_target, 0);
+
+	log_parse_category_mask(ringbuf_target, "DRLL:DCC");
+	log_parse_category_mask(ringbuf_target, "DRLL");
+	DEBUGP(DCC, "You should not see this\n");
+
+	log_parse_category_mask(ringbuf_target, "DRLL:DCC");
+	DEBUGP(DRLL, "You should see this\n");
+	DEBUGP(DCC, "You should see this\n");
+	DEBUGP(DMM, "You should not see this\n");
+	fprintf(stderr, ringbuffer_get_nth(ringbuf_target->tgt_rbvty.rb, 0));
+	fprintf(stderr, ringbuffer_get_nth(ringbuf_target->tgt_rbvty.rb, 1));
+	assert(!ringbuffer_get_nth(ringbuf_target->tgt_rbvty.rb, 2));
+
+	return 0;
+}