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/watchdog/Makefile b/drivers/watchdog/Makefile
index 7b39adc..1639d82 100644
--- a/drivers/watchdog/Makefile
+++ b/drivers/watchdog/Makefile
@@ -14,7 +14,7 @@
 obj-$(CONFIG_OMAP_WATCHDOG) += omap_wdt.o
 obj-$(CONFIG_DESIGNWARE_WATCHDOG) += designware_wdt.o
 obj-$(CONFIG_ULP_WATCHDOG) += ulp_wdog.o
-obj-$(CONFIG_$(SPL_TPL_)WDT) += wdt-uclass.o
+obj-$(CONFIG_$(PHASE_)WDT) += wdt-uclass.o
 obj-$(CONFIG_WDT_SANDBOX) += sandbox_wdt.o
 obj-$(CONFIG_WDT_ALARM_SANDBOX) += sandbox_alarm-wdt.o
 obj-$(CONFIG_WDT_APPLE) += apple_wdt.o