For that you'll have to repackage GraphHopper as there is no 'outside' configuration option provided - raise an issue if you need this.
Change the log4j.xml file under web/src/main/resources
e.g. replace the root and com.graphhopper logger via the following snippet:
<logger name="com.graphhopper" additivity="false">
<level value="warn" />
<appender-ref ref="ASYNC" />
</logger>
<root>
<priority value="warn"></priority>
<appender-ref ref="ASYNC"/>
</root>
Then repackage GH via the quickstart from source.
Update it might be possible without repackaging via
export JAVA_OPTS="-Dlog4j.debug -Dlog4j.configuration=some-log-config.xml"
using the mentioned and updated log4j.xml but I've not tested this yet. See the log4j docs