arm720t/clps7111: move serial driver to drivers/serial

add CONFIG_CLPS7111_SERIAL to activate the driver

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/arm720t/Makefile b/cpu/arm720t/Makefile
index a038042..d5ac7d3 100644
--- a/cpu/arm720t/Makefile
+++ b/cpu/arm720t/Makefile
@@ -26,7 +26,7 @@
 LIB	= $(obj)lib$(CPU).a
 
 START	= start.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 $(obj),$(COBJS) $(SOBJS))