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/Makefile b/Makefile
index d38adcb..ca74406 100644
--- a/Makefile
+++ b/Makefile
@@ -883,7 +883,7 @@
 ifdef CONFIG_POST
 libs-y += post/
 endif
-libs-$(CONFIG_$(SPL_TPL_)UNIT_TEST) += test/
+libs-$(CONFIG_$(PHASE_)UNIT_TEST) += test/
 libs-$(CONFIG_UT_ENV) += test/env/
 libs-$(CONFIG_UT_OPTEE) += test/optee/
 libs-$(CONFIG_UT_OVERLAY) += test/overlay/