ramlog: Add overwrite option to ramlog

Enable overwrite of circular buffer. If RAMLOG buffer overflows,
overwrite it from the top of buffer and always keep the latest log.

drivers/syslog/Kconfig

@@ -59,6 +59,13 @@ config RAMLOG_NPOLLWAITERS
 	---help---
 		The maximum number of threads that may be waiting on the poll method.
 
+config RAMLOG_OVERWRITE
+	bool "RAMLOG overwrite circular buffer"
+	default n
+	---help---
+		Enable overwrite of circular buffer. If RAMLOG buffer overflows,
+		overwrite it from the top of buffer and always keep the latest log.
+
 endif
 
 config DRIVER_NOTE

drivers/syslog/ramlog.c

@@ -227,10 +227,20 @@ static ssize_t ramlog_addchar(FAR struct ramlog_dev_s *priv, char ch)
 
   if (nexthead == priv->rl_tail)
     {
+#ifdef CONFIG_RAMLOG_OVERWRITE
+      /* Yes... Overwrite with the latest log in the circular buffer */
+
+      priv->rl_tail += 1;
+      if (priv->rl_tail >= priv->rl_bufsize)
+        {
+          priv->rl_tail = 0;
+        }
+#else
       /* Yes... Return an indication that nothing was saved in the buffer. */
 
       leave_critical_section(flags);
       return -EBUSY;
+#endif
     }
 
   /* No... copy the byte and re-enable interrupts */