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/x86/cpu/qemu/Makefile b/arch/x86/cpu/qemu/Makefile
index b7dd5bd..1439916 100644
--- a/arch/x86/cpu/qemu/Makefile
+++ b/arch/x86/cpu/qemu/Makefile
@@ -2,7 +2,7 @@
 #
 # Copyright (C) 2015, Bin Meng <bmeng.cn@gmail.com>
 
-ifndef CONFIG_$(SPL_)X86_64
+ifndef CONFIG_$(XPL_)X86_64
 obj-y += car.o
 endif
 obj-y += dram.o