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/mtd/Makefile b/drivers/mtd/Makefile
index db01041..ce05e20 100644
--- a/drivers/mtd/Makefile
+++ b/drivers/mtd/Makefile
@@ -34,9 +34,9 @@
 ifneq ($(mtd-y),)
 obj-$(CONFIG_SPL_MTD) += mtd.o
 endif
-obj-$(CONFIG_$(SPL_TPL_)NAND_SUPPORT) += nand/
+obj-$(CONFIG_$(PHASE_)NAND_SUPPORT) += nand/
 obj-$(CONFIG_SPL_ONENAND_SUPPORT) += onenand/
-obj-$(CONFIG_$(SPL_TPL_)SPI_FLASH_SUPPORT) += spi/
+obj-$(CONFIG_$(PHASE_)SPI_FLASH_SUPPORT) += spi/
 obj-$(CONFIG_SPL_UBI) += ubispl/
 
 endif