Update CFG_TEE_CORE_LOG_LEVEL flag in FVP

Fixes OP-TEE/optee_os#148

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>