added RAUC_KEYRING_FILE variable to handle custom certificate file name instead of ca.cert.pem
added RAUC_KEYRING_FILE variable to handle custom certificate file name instead of ca.cert.pem

Signed-off-by: Eugen Wiens <374f673d204a349d6a8c9400edb77137333c383d@jumo.net>

rauc/meta-rauc,rauc/meta-rauc,rauc/meta-rauc,rauc/meta-rauc