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/include/.gitignore b/include/.gitignore
index ef7dd5f..4481412 100644
--- a/include/.gitignore
+++ b/include/.gitignore
@@ -5,4 +5,5 @@
 /bmp_logo.h
 /config.h
 /config.mk
+/timestamp_autogenerated.h
 /version_autogenerated.h