x86: Use CONFIG_SERIAL_MULTI

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>
diff --git a/arch/i386/cpu/Makefile b/arch/i386/cpu/Makefile
index c658c6e..bb0a48f 100644
--- a/arch/i386/cpu/Makefile
+++ b/arch/i386/cpu/Makefile
@@ -29,7 +29,7 @@
 LIB	= $(obj)lib$(CPU).a
 
 START	= start.o start16.o resetvec.o
-COBJS	= serial.o interrupts.o cpu.o
+COBJS	= interrupts.o cpu.o
 
 SRCS	:= $(START:.o=.S) $(SOBJS:.o=.S) $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(SOBJS) $(COBJS))