Remove default value from SECURE_UPGRADE_DEV_SIGNING_KEY (sonic-net#1…

…4582)

This is done because when there is a default value, we mount to this path, and this creates this folder on the host.

#### Why I did it
Fix issue that running without overwriting SECURE_UPGRADE_DEV_SIGNING_KEY and SECURE_UPGRADE_DEV_SIGNING_CERT dummy folders are being created on the host.

#### How I did it
Removed the default assignment to SECURE_UPGRADE_DEV_SIGNING_KEY and SECURE_UPGRADE_DEV_SIGNING_CERT

#### How to verify it
Build SONiC using your own prod script

rules/config

@@ -223,8 +223,8 @@ SONIC_ENABLE_SECUREBOOT_SIGNATURE ?= n
 # SECURE_UPGRADE_DEV_SIGNING_CERT - path to development signing certificate, used for image signing during build
 # SECURE_UPGRADE_MODE - enum value for secure upgrade mode, valid options are "dev", "prod" and "no_sign"
 # SECURE_UPGRADE_PROD_SIGNING_TOOL - path to a vendor signing tool for production flow.
-SECURE_UPGRADE_DEV_SIGNING_KEY = /sonic/your/private/key/path/private_key.pem
-SECURE_UPGRADE_DEV_SIGNING_CERT =  /sonic/your/certificate/path/cert.pem
+SECURE_UPGRADE_DEV_SIGNING_KEY ?= 
+SECURE_UPGRADE_DEV_SIGNING_CERT ?= 
 SECURE_UPGRADE_MODE = "no_sign"
 SECURE_UPGRADE_PROD_SIGNING_TOOL ?=
 # PACKAGE_URL_PREFIX - the package url prefix