global: Rename SPL_ to XPL_

Use XPL_ as the symbol to indicate an SPL build. This means that SPL_ is
no-longer set.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/dfu/Makefile b/drivers/dfu/Makefile
index dfbf64d..05d7cc6 100644
--- a/drivers/dfu/Makefile
+++ b/drivers/dfu/Makefile
@@ -3,11 +3,11 @@
 # Copyright (C) 2012 Samsung Electronics
 # Lukasz Majewski <l.majewski@samsung.com>
 
-obj-$(CONFIG_$(SPL_)DFU) += dfu.o
-obj-$(CONFIG_$(SPL_)DFU_MMC) += dfu_mmc.o
-obj-$(CONFIG_$(SPL_)DFU_MTD) += dfu_mtd.o
-obj-$(CONFIG_$(SPL_)DFU_NAND) += dfu_nand.o
-obj-$(CONFIG_$(SPL_)DFU_RAM) += dfu_ram.o
-obj-$(CONFIG_$(SPL_)DFU_SF) += dfu_sf.o
-obj-$(CONFIG_$(SPL_)DFU_WRITE_ALT) += dfu_alt.o
-obj-$(CONFIG_$(SPL_)DFU_VIRT) += dfu_virt.o
+obj-$(CONFIG_$(XPL_)DFU) += dfu.o
+obj-$(CONFIG_$(XPL_)DFU_MMC) += dfu_mmc.o
+obj-$(CONFIG_$(XPL_)DFU_MTD) += dfu_mtd.o
+obj-$(CONFIG_$(XPL_)DFU_NAND) += dfu_nand.o
+obj-$(CONFIG_$(XPL_)DFU_RAM) += dfu_ram.o
+obj-$(CONFIG_$(XPL_)DFU_SF) += dfu_sf.o
+obj-$(CONFIG_$(XPL_)DFU_WRITE_ALT) += dfu_alt.o
+obj-$(CONFIG_$(XPL_)DFU_VIRT) += dfu_virt.o