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/adc/Makefile b/drivers/adc/Makefile
index dca0b39..665a796 100644
--- a/drivers/adc/Makefile
+++ b/drivers/adc/Makefile
@@ -4,7 +4,7 @@
 # Przemyslaw Marczak <p.marczak@samsung.com>
 #
 
-obj-$(CONFIG_$(SPL_TPL_)ADC) += adc-uclass.o
+obj-$(CONFIG_$(PHASE_)ADC) += adc-uclass.o
 obj-$(CONFIG_ADC_EXYNOS) += exynos-adc.o
 obj-$(CONFIG_ADC_SANDBOX) += sandbox.o
 obj-$(CONFIG_SARADC_ROCKCHIP) += rockchip-saradc.o