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/usb/host/Makefile b/drivers/usb/host/Makefile
index 07deb4f..792956e 100644
--- a/drivers/usb/host/Makefile
+++ b/drivers/usb/host/Makefile
@@ -8,8 +8,8 @@
 obj-$(CONFIG_SANDBOX) += usb-sandbox.o
 endif
 
-ifdef CONFIG_$(SPL_TPL_)USB_STORAGE
-obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += usb_bootdev.o
+ifdef CONFIG_$(PHASE_)USB_STORAGE
+obj-$(CONFIG_$(PHASE_)BOOTSTD) += usb_bootdev.o
 endif
 
 # ohci