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/scripts/Kbuild.include b/scripts/Kbuild.include
index b70b75e..1a6b28f 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -325,7 +325,7 @@
 .SECONDARY:
 
 ifdef CONFIG_XPL_BUILD
-SPL_ := SPL_
+XPL_ := SPL_
 ifeq ($(CONFIG_VPL_BUILD),y)
 SPL_TPL_ := VPL_
 else
@@ -336,6 +336,6 @@
 endif
 endif
 else
-SPL_ :=
+XPL_ :=
 SPL_TPL_ :=
 endif