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/leon2/start.S b/cpu/leon2/start.S
index 9b5d83e..b1f1eb5 100644
--- a/cpu/leon2/start.S
+++ b/cpu/leon2/start.S
@@ -27,6 +27,7 @@
 #include <asm/psr.h>
 #include <asm/stack.h>
 #include <asm/leon.h>
+#include <timestamp.h>
 #include <version.h>
 
 /* Entry for traps which jump to a programmer-specified trap handler.  */
@@ -199,7 +200,7 @@
 	.globl	version_string
 version_string:
 	.ascii U_BOOT_VERSION
-	.ascii " (", __DATE__, " - ", __TIME__, ")"
+	.ascii " (", U_BOOT_DATE, " - ", U_BOOT_TIME, ")"
 	.ascii CONFIG_IDENT_STRING, "\0"
 
 	.section	".text"