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/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 2738f0b..53a9490 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -19,7 +19,7 @@
 obj-$(CONFIG_CMD_BOOTM) += bootm.o
 endif
 obj-y	+= cmd_boot.o
-obj-$(CONFIG_$(SPL_)COREBOOT_SYSINFO)	+= coreboot/
+obj-$(CONFIG_$(XPL_)COREBOOT_SYSINFO)	+= coreboot/
 obj-$(CONFIG_SEABIOS) += coreboot_table.o
 obj-y	+= early_cmos.o
 obj-y	+= e820.o
@@ -90,7 +90,7 @@
 
 ifdef CONFIG_EFI_STUB
 
-ifeq ($(CONFIG_$(SPL_)X86_64),)
+ifeq ($(CONFIG_$(XPL_)X86_64),)
 extra-y += $(EFI_CRT0) $(EFI_RELOC)
 endif