config: add certs dir to CONFIG_MODULE_SIG_KEY

hulk inclusion
category: bugfix
bugzilla: NA
CVE: NA

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make[1]: *** No rule to make target 'signing_key.pem', needed by 'certs/signing_key.x509'.  Stop.
Fix the compile error by add certs dir to CONFIG_MODULE_SIG_KEY.

Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>