IIRC you can hit any ket to interupt uboot from the serial connection.
Then in uboot use printenv to see parameters, e.g. $this->bbcode_second_pass_code('', 'prinenv bootcmd') or $this->bbcode_second_pass_code('', 'printenv console').
Then to boot I guess you do something like $this->bbcode_second_pass_code('', 'run bootcmd')