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/board/ti/am43xx/Makefile b/board/ti/am43xx/Makefile
index b618f63..5b7b5b3 100644
--- a/board/ti/am43xx/Makefile
+++ b/board/ti/am43xx/Makefile
@@ -4,7 +4,7 @@
 #
 # Copyright (C) 2013 Texas Instruments Incorporated - https://www.ti.com/
 
-ifeq ($(CONFIG_$(SPL_)SKIP_LOWLEVEL_INIT),)
+ifeq ($(CONFIG_$(XPL_)SKIP_LOWLEVEL_INIT),)
 obj-y	:= mux.o
 endif