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/fs/sandbox/Makefile b/fs/sandbox/Makefile
index 880d59d..54ad842 100644
--- a/fs/sandbox/Makefile
+++ b/fs/sandbox/Makefile
@@ -9,4 +9,4 @@
 # Pavel Bartusek, Sysgo Real-Time Solutions AG, pba@sysgo.de
 
 obj-y := sandboxfs.o
-obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += host_bootdev.o
+obj-$(CONFIG_$(PHASE_)BOOTSTD) += host_bootdev.o