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/ecdsa/Makefile b/lib/ecdsa/Makefile
index 771d6d3..32b6183 100644
--- a/lib/ecdsa/Makefile
+++ b/lib/ecdsa/Makefile
@@ -1 +1 @@
-obj-$(CONFIG_$(SPL_)ECDSA_VERIFY) += ecdsa-verify.o
+obj-$(CONFIG_$(XPL_)ECDSA_VERIFY) += ecdsa-verify.o