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/arm/lib/Makefile b/arch/arm/lib/Makefile
index 1c95dd6..99989d6 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -96,7 +96,7 @@
 
 # some files can only build in ARM or THUMB2, not THUMB1
 
-ifdef CONFIG_$(XPL_)SYS_THUMB_BUILD
+ifdef CONFIG_$(PHASE_)SYS_THUMB_BUILD
 asflags-$(CONFIG_HAS_THUMB2) += -DCONFIG_THUMB2_KERNEL
 ifndef CONFIG_HAS_THUMB2