//devLog does unfortunately *not* get logged there
$GLOBALS['TYPO3_CONF_VARS']['LOG']['writerConfiguration'] = array(
    \TYPO3\CMS\Core\Log\LogLevel::DEBUG => array(
        'TYPO3\\CMS\\Core\\Log\\Writer\\FileWriter' => array(
            'logFile' => 'typo3temp/all.log'
        )
    )
);