Convert CONFIG_TIMESTAMP to Kconfig

This converts the following to Kconfig:
   CONFIG_TIMESTAMP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index f1ce576..c8d5906 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -25,6 +25,17 @@
 	  multiple configurations, verification through hashing and also
 	  verified boot (secure boot using RSA).
 
+config TIMESTAMP
+	bool "Show image date and time when displaying image information"
+	default y if CMD_DATE
+	help
+	  When CONFIG_TIMESTAMP is selected, the timestamp (date and time) of
+	  an image is printed by image commands like bootm or iminfo. This
+	  is shown as 'Timestamp: xxx' and 'Created: xxx'. If this option is
+	  enabled, then U-Boot requires FITs to have a timestamp. If a FIT is
+	  loaded that does not, the message 'Wrong FIT format: no timestamp'
+	  is shown.
+
 if FIT
 
 config FIT_EXTERNAL_OFFSET