imgtool: dumpinfo: Avoid exit and use return

Signed-off-by: Guillaume G. <guillaume.galeazzi+gh@gmail.com>
diff --git a/scripts/imgtool/dumpinfo.py b/scripts/imgtool/dumpinfo.py
index 5544657..40ece79 100644
--- a/scripts/imgtool/dumpinfo.py
+++ b/scripts/imgtool/dumpinfo.py
@@ -251,7 +251,7 @@
     ###############################################################################
 
     if silent:
-        sys.exit(0)
+        return
 
     print("Printing content of signed image:", os.path.basename(imgfile), "\n")