Obtained from:
sio.c and sioreg.c changed to allow autodetecting the RB_SERIAL flag
passed by the boot blocks so that the kernel can switch to 'serial
console' mode automagically. 'options COMCONSOLE' can still be specified
to force the kernel to always use the serial port as a console.
CONUNIT and CONADDR can also be specified in the kernel config file
if the user wants to shift the console to a different port.
Poul-Henning Kamp
Bill Paul
Rodney W. Grimes