Convert CONFIG_TRACE to Kconfig

This converts the following to Kconfig:
   CONFIG_TRACE

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index 2120216..a3352a4 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -165,6 +165,15 @@
 config BITREVERSE
 	bool "Bit reverse library from Linux"
 
+config TRACE
+	bool "Support for tracing of function calls and timing"
+	imply CMD_TRACE
+	help
+	  Enables function tracing within U-Boot. This allows recording of call
+	  traces including timing information. The command can write data to
+	  memory for exporting for analysis (e.g. using bootchart).
+	  See doc/README.trace for full details.
+
 source lib/dhry/Kconfig
 
 menu "Security support"