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/cmd/Makefile b/cmd/Makefile
index ed62e1c..21d3763 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -249,7 +249,7 @@
 obj-$(CONFIG_ARCH_OMAP2PLUS) += ti/
 endif # !CONFIG_XPL_BUILD
 
-obj-$(CONFIG_$(SPL_)CMD_TLV_EEPROM) += tlv_eeprom.o
+obj-$(CONFIG_$(XPL_)CMD_TLV_EEPROM) += tlv_eeprom.o
 
 obj-$(CONFIG_CMD_BCM_EXT_UTILS) += broadcom/