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/common/init/Makefile b/common/init/Makefile
index 853b56d..224e092 100644
--- a/common/init/Makefile
+++ b/common/init/Makefile
@@ -5,4 +5,4 @@
 #
 
 obj-y += board_init.o
-obj-$(CONFIG_$(SPL_TPL_)HANDOFF) += handoff.o
+obj-$(CONFIG_$(PHASE_)HANDOFF) += handoff.o