sandbox: Enable the 'trace' command when tracing is used

Enable this by default so that tracing can be inspected if enabled. This
cannot rely on the 'imply' in lib/Kconfig since this method of enabling
tracing relates on an environment variable (FTRACE) and does not use
Kconfig.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index 6dd03e3..bf03bae 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -8,11 +8,11 @@
 
 #ifdef FTRACE
 #define CONFIG_TRACE
+#define CONFIG_CMD_TRACE
 #define CONFIG_TRACE_BUFFER_SIZE	(16 << 20)
 #define CONFIG_TRACE_EARLY_SIZE		(16 << 20)
 #define CONFIG_TRACE_EARLY
 #define CONFIG_TRACE_EARLY_ADDR		0x00100000
-
 #endif
 
 #ifndef CONFIG_SPL_BUILD