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/disk/Makefile b/disk/Makefile
index 45588cf..2248a65 100644
--- a/disk/Makefile
+++ b/disk/Makefile
@@ -5,16 +5,16 @@
 
 #ccflags-y += -DET_DEBUG -DDEBUG
 
-obj-$(CONFIG_$(SPL_TPL_)PARTITIONS)  += part.o
-ifdef CONFIG_$(SPL_TPL_)BLK
-obj-$(CONFIG_$(SPL_TPL_)PARTITIONS)  += disk-uclass.o
+obj-$(CONFIG_$(PHASE_)PARTITIONS)  += part.o
+ifdef CONFIG_$(PHASE_)BLK
+obj-$(CONFIG_$(PHASE_)PARTITIONS)  += disk-uclass.o
 endif
 
 # Must have BLK or SPL_LEGACY_BLOCK to support partitions
-ifneq ($(CONFIG_$(SPL_TPL_)BLK),$(CONFIG_SPL_LEGACY_BLOCK),)
-obj-$(CONFIG_$(SPL_TPL_)MAC_PARTITION)   += part_mac.o
-obj-$(CONFIG_$(SPL_TPL_)DOS_PARTITION)   += part_dos.o
-obj-$(CONFIG_$(SPL_TPL_)ISO_PARTITION)   += part_iso.o
-obj-$(CONFIG_$(SPL_TPL_)AMIGA_PARTITION) += part_amiga.o
-obj-$(CONFIG_$(SPL_TPL_)EFI_PARTITION)   += part_efi.o
+ifneq ($(CONFIG_$(PHASE_)BLK),$(CONFIG_SPL_LEGACY_BLOCK),)
+obj-$(CONFIG_$(PHASE_)MAC_PARTITION)   += part_mac.o
+obj-$(CONFIG_$(PHASE_)DOS_PARTITION)   += part_dos.o
+obj-$(CONFIG_$(PHASE_)ISO_PARTITION)   += part_iso.o
+obj-$(CONFIG_$(PHASE_)AMIGA_PARTITION) += part_amiga.o
+obj-$(CONFIG_$(PHASE_)EFI_PARTITION)   += part_efi.o
 endif