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/lib/acpi/Makefile b/lib/acpi/Makefile
index 99be19a..dcca0c6 100644
--- a/lib/acpi/Makefile
+++ b/lib/acpi/Makefile
@@ -3,7 +3,7 @@
 
 obj-y += acpi.o
 
-ifdef CONFIG_$(SPL_TPL_)GENERATE_ACPI_TABLE
+ifdef CONFIG_$(PHASE_)GENERATE_ACPI_TABLE
 
 obj-$(CONFIG_$(XPL_)ACPIGEN) += acpigen.o
 obj-$(CONFIG_$(XPL_)ACPIGEN) += acpi_device.o