The Petitboot kernel contains a default command line set in the skiroot kernel configuration:

CONFIG_CMDLINE="console=tty0 console=hvc0 ipr.fast_reboot=1 quiet"

You can add options to this by setting a nvram variable:

nvram -p ibm,skiboot --update-config bootargs="xmon=on"

This can also be used for blacklisting problematic kernel drivers, or enabling extra debugging when chasing down a bootloader kernel issue.