Enabling the serial console by default allows getting boot output
easier when running on virtualization platforms that have support for
a serial port. The video console is left enabled in multi console
mode, so environments without a serial console shouldn't notice any
change.
Note that the speed is hardcoded to 115200, all virtualization
platforms should support the highest possible speed, since it's using
an emulated serial port that's not tied to any specific hardware.
Sponsored by: Citrix Systems R&D