tools: add boot/ to .gitignore

/tools/boot/ is a build product. Add it to .gitignore

Signed-off-by: Du Huanpeng <dhu@hodcarrier.org>
diff --git a/tools/.gitignore b/tools/.gitignore
index a88453f..d3a93ff 100644
--- a/tools/.gitignore
+++ b/tools/.gitignore
@@ -36,3 +36,4 @@
 /update_octeon_header
 /version.h
 /xway-swap-bytes
+/boot