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/core/Makefile b/drivers/core/Makefile
index acbd2bf..20dbff6 100644
--- a/drivers/core/Makefile
+++ b/drivers/core/Makefile
@@ -6,16 +6,16 @@
 obj-$(CONFIG_$(SPL_TPL_)ACPIGEN) += acpi.o
 obj-$(CONFIG_$(SPL_TPL_)DEVRES) += devres.o
 obj-$(CONFIG_$(SPL_TPL_)DM_DEVICE_REMOVE)	+= device-remove.o
-obj-$(CONFIG_$(SPL_)SIMPLE_BUS)	+= simple-bus.o
+obj-$(CONFIG_$(XPL_)SIMPLE_BUS)	+= simple-bus.o
 obj-$(CONFIG_SIMPLE_PM_BUS)	+= simple-pm-bus.o
 obj-$(CONFIG_DM)	+= dump.o
 obj-$(CONFIG_$(SPL_TPL_)REGMAP)	+= regmap.o
 obj-$(CONFIG_$(SPL_TPL_)SYSCON)	+= syscon-uclass.o
-obj-$(CONFIG_$(SPL_)OF_LIVE) += of_access.o of_addr.o
+obj-$(CONFIG_$(XPL_)OF_LIVE) += of_access.o of_addr.o
 ifndef CONFIG_DM_DEV_READ_INLINE
 obj-$(CONFIG_OF_CONTROL) += read.o
 endif
-obj-$(CONFIG_$(SPL_)OF_PLATDATA) += read.o
+obj-$(CONFIG_$(XPL_)OF_PLATDATA) += read.o
 obj-$(CONFIG_OF_CONTROL) += of_extra.o ofnode.o read_extra.o
 
 ccflags-$(CONFIG_DM_DEBUG) += -DDEBUG