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/lib/aes/Makefile b/lib/aes/Makefile
index daed52a..ad6228a 100644
--- a/lib/aes/Makefile
+++ b/lib/aes/Makefile
@@ -2,4 +2,4 @@
 #
 # Copyright (c) 2019, Softathome
 
-obj-$(CONFIG_$(SPL_)FIT_CIPHER) += aes-decrypt.o
+obj-$(CONFIG_$(XPL_)FIT_CIPHER) += aes-decrypt.o