serial: Unconditionally enable CONFIG_SERIAL_MULTI

Enable CONFIG_SERIAL_MULTI for all builds of U-Boot. That includes
both SPL builds and non-SPL builds, everything. To avoid poluting
this patch with removal of ifdef-endif constructions containing
CONFIG_SERIAL_MULTI, the CONFIG_SERIAL_MULTI is temporarily added
into CPPFLAGS in config.mk . This will be again removed in following
patch.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>

config.mk

@@ -194,7 +194,7 @@ OBJCFLAGS += --gap-fill=0xff
 gccincdir := $(shell $(CC) -print-file-name=include)
 
 CPPFLAGS := $(DBGFLAGS) $(OPTFLAGS) $(RELFLAGS)		\
-	-D__KERNEL__
+	-D__KERNEL__ -DCONFIG_SERIAL_MULTI
 
 # Enable garbage collection of un-used sections for SPL
 ifeq ($(CONFIG_SPL_BUILD),y)

drivers/serial/Makefile

@@ -25,9 +25,7 @@ include $(TOPDIR)/config.mk
 
 LIB	:= $(obj)libserial.o
 
-ifndef CONFIG_SPL_BUILD
 COBJS-$(CONFIG_SERIAL_MULTI) += serial.o
-endif
 
 COBJS-$(CONFIG_ALTERA_UART) += altera_uart.o
 COBJS-$(CONFIG_ALTERA_JTAG_UART) += altera_jtag_uart.o