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/test/fuzz/Makefile b/test/fuzz/Makefile
index 663b79c..8a135b6 100644
--- a/test/fuzz/Makefile
+++ b/test/fuzz/Makefile
@@ -4,5 +4,5 @@
 # Written by Andrew Scull <ascull@google.com>
 #
 
-obj-$(CONFIG_$(SPL_)CMDLINE) += cmd_fuzz.o
+obj-$(CONFIG_$(XPL_)CMDLINE) += cmd_fuzz.o
 obj-$(CONFIG_VIRTIO_SANDBOX) += virtio.o