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>

scripts/Kbuild.include

@@ -327,15 +327,15 @@ endif
 ifdef CONFIG_XPL_BUILD
 XPL_ := SPL_
 ifeq ($(CONFIG_VPL_BUILD),y)
-SPL_TPL_ := VPL_
+PHASE_ := VPL_
 else
 ifeq ($(CONFIG_TPL_BUILD),y)
-SPL_TPL_ := TPL_
+PHASE_ := TPL_
 else
-SPL_TPL_ := SPL_
+PHASE_ := SPL_
 endif
 endif
 else
 XPL_ :=
-SPL_TPL_ :=
+PHASE_ :=
 endif