log: Add a test command

Add a command which exercises the logging system.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index b745a7e..c033223 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1507,7 +1507,8 @@
 	help
 	  This provides access to logging features. It allows the output of
 	  log data to be controlled to a limited extent (setting up the default
-	  maximum log level for emitting of records).
+	  maximum log level for emitting of records). It also provides access
+	  to a command used for testing the log system.
 
 config CMD_TRACE
 	bool "trace - Support tracing of function calls and timing"