trace: sandbox: Use only the Kconfig options

At present there are Kconfig options for tracing, but sandbox uses
plain #defines to set them. Correct this and make the tracing command
default to enabled so that this is not needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/trace.rst b/doc/develop/trace.rst
index 09f5745..b22e068 100644
--- a/doc/develop/trace.rst
+++ b/doc/develop/trace.rst
@@ -30,16 +30,11 @@
 way of trying out tracing before you use it on your actual board. To do
 this, follow these steps:
 
-Add the following to include/configs/sandbox.h (if not already there)
+Add the following to config/sandbox_defconfig
 
 .. code-block:: c
 
-    #define CONFIG_TRACE
-    #define CONFIG_CMD_TRACE
-    #define CONFIG_TRACE_BUFFER_SIZE    (16 << 20)
-    #define CONFIG_TRACE_EARLY_SIZE     (8 << 20)
-    #define CONFIG_TRACE_EARLY
-    #define CONFIG_TRACE_EARLY_ADDR     0x00100000
+    CONFIG_TRACE=y
 
 Build sandbox U-Boot with tracing enabled: