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/riscv/Makefile b/arch/riscv/Makefile
index c36a853..4e6f347 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -48,7 +48,7 @@
 ARCH_FLAGS = -march=$(RISCV_MARCH) -mabi=$(ABI) \
 	     -mcmodel=$(CMODEL)
 
-ifeq ($(CONFIG_$(SPL_)FRAMEPOINTER),y)
+ifeq ($(CONFIG_$(XPL_)FRAMEPOINTER),y)
 	ARCH_FLAGS += -fno-omit-frame-pointer
 endif