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/arch/arm/mach-omap2/am33xx/Makefile b/arch/arm/mach-omap2/am33xx/Makefile
index 3f1af7f..6f3587f 100644
--- a/arch/arm/mach-omap2/am33xx/Makefile
+++ b/arch/arm/mach-omap2/am33xx/Makefile
@@ -11,7 +11,7 @@
 
 obj-y	+= sys_info.o
 obj-y	+= ddr.o
-ifeq ($(CONFIG_$(SPL_)SKIP_LOWLEVEL_INIT),)
+ifeq ($(CONFIG_$(XPL_)SKIP_LOWLEVEL_INIT),)
 obj-y	+= emif4.o
 endif
 obj-y	+= board.o