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/tools/qconfig.py b/tools/qconfig.py
index e025f56..7b4934b 100755
--- a/tools/qconfig.py
+++ b/tools/qconfig.py
@@ -1222,8 +1222,8 @@
     (None, 'FRED')
     >>> RE_MK_CONFIGS.search('CONFIG_$(XPL_)MARY').groups()
     ('$(XPL_)', 'MARY')
-    >>> RE_MK_CONFIGS.search('CONFIG_$(SPL_TPL_)MARY').groups()
-    ('$(SPL_TPL_)', 'MARY')
+    >>> RE_MK_CONFIGS.search('CONFIG_$(PHASE_)MARY').groups()
+    ('$(PHASE_)', 'MARY')
     """
     all_uses = collections.defaultdict(list)
     fname_uses = {}
@@ -1322,7 +1322,7 @@
                 an SPL_ option, but for which there is none;
                 for MOD_PROPER, look at source code which implies a Proper
                 option (i.e. use of CONFIG_IS_ENABLED() or $(XPL_) or
-                $(SPL_TPL_) but for which there none;
+                $(PHASE_) but for which there none;
                 if MODE_NORMAL, ignore SPL
 
         Returns: