Kbuild: Always use $(PHASE_)

It is confusing to have both "$(PHASE_)" and "$(XPL_)" be used in our
Makefiles as part of the macros to determine when to do something in our
Makefiles based on what phase of the build we are in. For consistency,
bring this down to a single macro and use "$(PHASE_)" only.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/x86/cpu/qemu/Makefile b/arch/x86/cpu/qemu/Makefile
index 1439916..2134b0d 100644
--- a/arch/x86/cpu/qemu/Makefile
+++ b/arch/x86/cpu/qemu/Makefile
@@ -2,7 +2,7 @@
 #
 # Copyright (C) 2015, Bin Meng <bmeng.cn@gmail.com>
 
-ifndef CONFIG_$(XPL_)X86_64
+ifndef CONFIG_$(PHASE_)X86_64
 obj-y += car.o
 endif
 obj-y += dram.o