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/cmd/Kconfig b/cmd/Kconfig
index 5b30b13..fd8f022 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -2350,6 +2350,8 @@
 
 config CMD_TRACE
 	bool "trace - Support tracing of function calls and timing"
+	depends on TRACE
+	default y
 	help
 	  Enables a command to control using of function tracing within
 	  U-Boot. This allows recording of call traces including timing