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/drivers/scsi/Makefile b/drivers/scsi/Makefile
index 7c19a43..b76de1b 100644
--- a/drivers/scsi/Makefile
+++ b/drivers/scsi/Makefile
@@ -6,7 +6,7 @@
 ifndef CONFIG_XPL_BUILD
 obj-$(CONFIG_SCSI) += scsi.o scsi-uclass.o
 ifdef CONFIG_SCSI
-obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += scsi_bootdev.o
+obj-$(CONFIG_$(PHASE_)BOOTSTD) += scsi_bootdev.o
 obj-$(CONFIG_SANDBOX) += sandbox_scsi.o
 obj-$(CONFIG_SANDBOX) += scsi_emul.o
 endif