[comboot] Avoid dragging in serial console support unconditionally

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>

src/arch/i386/interface/syslinux/comboot_call.c

 /* Mode flags set by INT 22h AX=0017h */
 static uint16_t comboot_graphics_mode = 0;
 
-
 /**
  * Print a string with a particular terminator
  */
 	unhook_bios_interrupt ( 0x22, ( unsigned int ) int22_wrapper,
 				&int22_vector );
 }
+
+/* Avoid dragging in serial console support unconditionally */
+struct uart serial_console __attribute__ (( weak ));