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/drivers/serial/Makefile b/drivers/serial/Makefile
index 1c0b013..ebe692a 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -3,7 +3,7 @@
 # (C) Copyright 2006-2009
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-ifeq ($(CONFIG_$(SPL_TPL_)DM_SERIAL),y)
+ifeq ($(CONFIG_$(PHASE_)DM_SERIAL),y)
 obj-y += serial-uclass.o
 else
 obj-y += serial.o