tools, fit_check_sign: verify a signed fit image

add host tool "fit_check_sign" which verifies, if a fit image is
signed correct.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Simon Glass <sjg@chromium.org>
diff --git a/tools/.gitignore b/tools/.gitignore
index 2f6ecc7..b1e997f 100644
--- a/tools/.gitignore
+++ b/tools/.gitignore
@@ -1,5 +1,6 @@
 /bmp_logo
 /envcrc
+/fit_check_sign
 /fit_info
 /gen_eth_addr
 /img2srec