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/lib_mips/board.c b/lib_mips/board.c
index 77e1cc8..9c997f1 100644
--- a/lib_mips/board.c
+++ b/lib_mips/board.c
@@ -25,6 +25,7 @@
 #include <command.h>
 #include <malloc.h>
 #include <devices.h>
+#include <timestamp.h>
 #include <version.h>
 #include <net.h>
 #include <environment.h>
@@ -53,7 +54,7 @@
 ulong monitor_flash_len;
 
 const char version_string[] =
-	U_BOOT_VERSION" (" __DATE__ " - " __TIME__ ")";
+	U_BOOT_VERSION" (" U_BOOT_DATE " - " U_BOOT_TIME ")";
 
 static char *failed = "*** failed ***\n";