You can define "console devices" using the console= boot argument.
Add something like this to your bootargs setting:
... console=tty0 console=ttyS0,$(baudrate) ...
This will ensure that the boot messages are displayed on both the
framebuffer (/dev/tty0) and the serial
console (/dev/ttyS0); the last device named in a
console= option will be the one that takes input,
too, so with the settings above you can use the serial console to
enter commands etc.
For a more detailed description see http://www.tldp.org/HOWTO/Remote-Serial-Console-HOWTO/configure-kernel.html
