Secure Boot: Default signing mode to development

- Defaulted signing mode to development in openpower-pnor config
diff --git a/openpower/package/openpower-pnor/Config.in b/openpower/package/openpower-pnor/Config.in
index 644d281..37c4470 100644
--- a/openpower/package/openpower-pnor/Config.in
+++ b/openpower/package/openpower-pnor/Config.in
@@ -39,6 +39,7 @@
 
 config BR2_OPENPOWER_SECUREBOOT_SIGN_MODE
         string "Secureboot signing mode"
+        default "development"
         help
             Available options [development | production]
             Indicates the signing mode when generating the PNOR image.  Only