When the ability for iPXE to handle multiple serial ports was added,
the choice was made that the singular serial port referred to by
COMBOOT calls should mean the port used for the serial console.  This
unintentionally caused IMAGE_COMBOOT to also enable CONSOLE_SERIAL.

Fix by providing a weak-symbol version of the serial console which
will be used if serial console support was not explicitly enabled.

Reported-by: Torgeir Wulfsberg <Torgeir.Wulfsberg@kongsberg.com>
Reported-by: Ján ONDREJ (SAL) <ondrejj@salstar.sk>
Signed-off-by: Michael Brown <mcb30@ipxe.org>