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/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index f797f8c..c0caa70 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -46,5 +46,5 @@
 endif # not minimal
 
 ifdef CONFIG_XPL_BUILD
-obj-$(CONFIG_$(SPL_TPL)_FRAMEWORK) += spl.o
+obj-$(CONFIG_$(PHASE_)FRAMEWORK) += spl.o
 endif