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/drivers/nvme/Makefile b/drivers/nvme/Makefile
index fd3e68a..8c32cfb 100644
--- a/drivers/nvme/Makefile
+++ b/drivers/nvme/Makefile
@@ -4,4 +4,4 @@
 
 obj-y += nvme-uclass.o nvme.o nvme_show.o
 obj-$(CONFIG_NVME_APPLE) += nvme_apple.o
-obj-$(CONFIG_$(SPL_)NVME_PCI) += nvme_pci.o
+obj-$(CONFIG_$(XPL_)NVME_PCI) += nvme_pci.o