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/cpu/arm926ejs/Makefile b/arch/arm/cpu/arm926ejs/Makefile
index 7551388..750cb94 100644
--- a/arch/arm/cpu/arm926ejs/Makefile
+++ b/arch/arm/cpu/arm926ejs/Makefile
@@ -17,7 +17,7 @@
 
 # some files can only build in ARM or THUMB2, not THUMB1
 
-ifdef CONFIG_$(SPL_)SYS_THUMB_BUILD
+ifdef CONFIG_$(XPL_)SYS_THUMB_BUILD
 ifndef CONFIG_HAS_THUMB2
 
 CFLAGS_cpu.o := -marm