global: Rename SPL_TPL_ to PHASE_

Use PHASE_ as the symbol to select a particular XPL build. This means
that SPL_TPL_ is no-longer set.

Update the comment in bootstage to refer to this symbol, instead of
SPL_

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/libfdt/Makefile b/lib/libfdt/Makefile
index 1fe50ec..c492377 100644
--- a/lib/libfdt/Makefile
+++ b/lib/libfdt/Makefile
@@ -16,4 +16,4 @@
 obj-$(CONFIG_OF_LIBFDT_OVERLAY) += fdt_overlay.o
 
 ccflags-y := -I$(srctree)/scripts/dtc/libfdt \
-	-DFDT_ASSUME_MASK=$(CONFIG_$(SPL_TPL_)OF_LIBFDT_ASSUME_MASK)
+	-DFDT_ASSUME_MASK=$(CONFIG_$(PHASE_)OF_LIBFDT_ASSUME_MASK)