Update U-Boot's build timestamp on every compile

Use the GNU 'date' command to auto-generate a new U-Boot
timestamp on every compile.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/cpu/mcf5445x/start.S b/cpu/mcf5445x/start.S
index 61e43ff..d5a7f93 100644
--- a/cpu/mcf5445x/start.S
+++ b/cpu/mcf5445x/start.S
@@ -22,6 +22,7 @@
  */
 
 #include <config.h>
+#include <timestamp.h>
 #include "version.h"
 
 #ifndef	 CONFIG_IDENT_STRING
@@ -627,6 +628,6 @@
 	.globl	version_string
 version_string:
 	.ascii U_BOOT_VERSION
-	.ascii " (", __DATE__, " - ", __TIME__, ")"
+	.ascii " (", U_BOOT_DATE, " - ", U_BOOT_TIME, ")"
 	.ascii CONFIG_IDENT_STRING, "\0"
 	.align 4