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/gpio/Makefile b/drivers/gpio/Makefile
index 24d1b03..21b4325 100644
--- a/drivers/gpio/Makefile
+++ b/drivers/gpio/Makefile
@@ -8,7 +8,7 @@
 obj-$(CONFIG_AXP_GPIO)		+= axp_gpio.o
 obj-$(CONFIG_DM_74X164)		+= 74x164_gpio.o
 endif
-obj-$(CONFIG_$(SPL_TPL_)DM_GPIO) += gpio-uclass.o
+obj-$(CONFIG_$(PHASE_)DM_GPIO) += gpio-uclass.o
 
 obj-$(CONFIG_$(XPL_)DM_PCA953X)	+= pca953x_gpio.o
 
@@ -24,7 +24,7 @@
 obj-$(CONFIG_INTEL_BROADWELL_GPIO)	+= intel_broadwell_gpio.o
 obj-$(CONFIG_IPROC_GPIO)	+= iproc_gpio.o
 obj-$(CONFIG_KIRKWOOD_GPIO)	+= kw_gpio.o
-obj-$(CONFIG_$(SPL_TPL_)MCP230XX_GPIO)	+= mcp230xx_gpio.o
+obj-$(CONFIG_$(PHASE_)MCP230XX_GPIO)	+= mcp230xx_gpio.o
 obj-$(CONFIG_MXC_GPIO)	+= mxc_gpio.o
 obj-$(CONFIG_MXS_GPIO)	+= mxs_gpio.o
 obj-$(CONFIG_NPCM_GPIO)      	+= npcm_gpio.o
@@ -63,7 +63,7 @@
 obj-$(CONFIG_MVEBU_GPIO)	+= mvebu_gpio.o
 obj-$(CONFIG_MSM_GPIO)		+= msm_gpio.o
 obj-$(CONFIG_$(XPL_)PCF8575_GPIO)	+= pcf8575_gpio.o
-obj-$(CONFIG_$(SPL_TPL_)QCOM_PMIC_GPIO)	+= qcom_pmic_gpio.o
+obj-$(CONFIG_$(PHASE_)QCOM_PMIC_GPIO)	+= qcom_pmic_gpio.o
 obj-$(CONFIG_MT7620_GPIO)	+= mt7620_gpio.o
 obj-$(CONFIG_MT7621_GPIO)	+= mt7621_gpio.o
 obj-$(CONFIG_MSCC_SGPIO)	+= mscc_sgpio.o