how to pass log level to 7 from u-boot

Hi You can change log level in linux runtime echo 7 > /proc/sys/kernel/printk however my device hangs on fastly after I am able to write anything through serial console. Does anybody of You know how to pass it from u-boot? I can stop in u-boot and pass log level 7 but I don't know what command should I write in order to do it. BR Tomek