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/ppc4xx/start.S b/cpu/ppc4xx/start.S
index e68cf9b..4b5349e 100644
--- a/cpu/ppc4xx/start.S
+++ b/cpu/ppc4xx/start.S
@@ -63,6 +63,7 @@
  */
 #include <config.h>
 #include <ppc4xx.h>
+#include <timestamp.h>
 #include <version.h>
 
 #define _LINUX_CONFIG_H 1	/* avoid reading Linux autoconf.h file	*/
@@ -510,7 +511,7 @@
 	.globl	version_string
 version_string:
 	.ascii U_BOOT_VERSION
-	.ascii " (", __DATE__, " - ", __TIME__, ")"
+	.ascii " (", U_BOOT_DATE, " - ", U_BOOT_TIME, ")"
 	.ascii CONFIG_IDENT_STRING, "\0"
 
 	. = EXC_OFF_SYS_RESET