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/rng/Makefile b/drivers/rng/Makefile
index 30553c9..30c5827 100644
--- a/drivers/rng/Makefile
+++ b/drivers/rng/Makefile
@@ -3,7 +3,7 @@
 # Copyright (c) 2019, Linaro Limited
 #
 
-obj-$(CONFIG_$(SPL_TPL_)DM_RNG) += rng-uclass.o
+obj-$(CONFIG_$(PHASE_)DM_RNG) += rng-uclass.o
 obj-$(CONFIG_RNG_MESON) += meson-rng.o
 obj-$(CONFIG_RNG_SANDBOX) += sandbox_rng.o
 obj-$(CONFIG_RNG_MSM) += msm_rng.o