Allow global overriding LOCAL_JACK_ENABLED

If the variable ANDROID_FORCE_JACK_ENABLED it will override any local
value.

Change-Id: I3fc99b97783dd3a89c3240a8a4269ebf9e477f3f
diff --git a/core/configure_local_jack.mk b/core/configure_local_jack.mk
index 159fac4..297c7e2 100644
--- a/core/configure_local_jack.mk
+++ b/core/configure_local_jack.mk
@@ -14,6 +14,9 @@
 # limitations under the License.
 #
 
+ifdef ANDROID_FORCE_JACK_ENABLED
+LOCAL_JACK_ENABLED := $(ANDROID_FORCE_JACK_ENABLED)
+endif
 LOCAL_JACK_ENABLED := $(strip $(LOCAL_JACK_ENABLED))
 ifneq ($(LOCAL_JACK_ENABLED),full)
 ifneq ($(LOCAL_JACK_ENABLED),incremental)