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/arch/x86/Makefile b/arch/x86/Makefile
index bc1e1ed..fd409b9 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -4,7 +4,7 @@
 ifdef CONFIG_$(XPL_)X86_64
 head-y := arch/x86/cpu/start64.o
 else
-ifeq ($(CONFIG_$(SPL_TPL_)X86_16BIT_INIT),y)
+ifeq ($(CONFIG_$(PHASE_)X86_16BIT_INIT),y)
 head-y := arch/x86/cpu/start.o
 else
 ifndef CONFIG_SPL
@@ -20,8 +20,8 @@
 endif
 endif # EFI
 
-head-$(CONFIG_$(SPL_TPL_)X86_16BIT_INIT) += arch/x86/cpu/start16.o
-head-$(CONFIG_$(SPL_TPL_)X86_16BIT_INIT) += arch/x86/cpu/resetvec.o
+head-$(CONFIG_$(PHASE_)X86_16BIT_INIT) += arch/x86/cpu/start16.o
+head-$(CONFIG_$(PHASE_)X86_16BIT_INIT) += arch/x86/cpu/resetvec.o
 
 libs-y += arch/x86/cpu/
 libs-y += arch/x86/lib/