Make ads5121 out-of-tree compiling safe

Reuse the existing DIU driver in board/freescale/common.
Signed-off-by: NYork Sun <yorksun@freescale.com>

board/ads5121/Makefile

@@ -23,9 +23,14 @@
 
 include $(TOPDIR)/config.mk
 
+$(shell mkdir -p $(OBJTREE)/board/freescale/common)
+
 LIB	= $(obj)lib$(BOARD).a
 
-COBJS-y	:= $(BOARD).o ads5121_diu.o ../freescale/common/fsl_diu_fb.o ../freescale/common/fsl_logo_bmp.o
+COBJS-y	:= $(BOARD).o
+COBJS-${CONFIG_FSL_DIU_FB} += ads5121_diu.o
+COBJS-${CONFIG_FSL_DIU_FB} += ../freescale/common/fsl_diu_fb.o
+COBJS-${CONFIG_FSL_DIU_FB} += ../freescale/common/fsl_logo_bmp.o
 COBJS-$(CONFIG_PCI) += pci.o
 
 COBJS	:= $(COBJS-y)