art: build: Fix defines for USE_DEX2OAT_DEBUG

* "?=" cannot be overriden by "?="

Change-Id: Ib5184d98740075e38375a2a7c27f117776717f33
Signed-off-by: Alex Naidis <alex.naidis@linux.com>
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index a057d6d..c6eeaa9 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -30,13 +30,14 @@
 # Beware that tests may use the non-debug build for performance, notable 055-enum-performance
 #
 ART_BUILD_TARGET_NDEBUG ?= true
-ART_BUILD_TARGET_DEBUG ?= false
 ART_BUILD_HOST_NDEBUG ?= true
-ART_BUILD_HOST_DEBUG ?= false
 
 ifneq ($(USE_DEX2OAT_DEBUG),false)
 ART_BUILD_TARGET_DEBUG ?= true
 ART_BUILD_HOST_DEBUG ?= true
+else
+ART_BUILD_TARGET_DEBUG ?= false
+ART_BUILD_HOST_DEBUG ?= false
 endif
 
 # Set this to change what opt level Art is built at.