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>

scripts/Makefile.xpl

@@ -59,7 +59,7 @@ endif
 export SPL_NAME
 
 ifdef CONFIG_XPL_BUILD
-SPL_ := SPL_
+XPL_ := SPL_
 ifeq ($(CONFIG_VPL_BUILD),y)
 SPL_TPL_ := VPL_
 else
@@ -70,7 +70,7 @@ SPL_TPL_ := SPL_
 endif
 endif
 else
-SPL_ :=
+XPL_ :=
 SPL_TPL_ :=
 endif