Fix kernel-log clobbering serial console

[Rahix]

If kernel-logging is set to be verbose, kernel log-messages will clobber the serial console and lead to incorrect test results or prevent proper board initialization.

tbot should at least be able to acquire the shell and maybe provide a config option that allows turning off kernel logging via sysctl.

[Rahix]

To fix this issue, you can temporarily add loglevel=2 to your kernel commandline.