Don't overwrite PRODUCT_PACKAGES_DEBUG

We're getting useful debug utilities included by base_system.mk but
we're overwriting the variable so tools like su and remount are not
getting built. Fix this by extending the variable instead of overwriting
it.

Change-Id: Ieabdd1b6bd2cf2cfed975f9d74b9b385ae58197e
diff --git a/device.mk b/device.mk
index 8779b82..5d7866e 100644
--- a/device.mk
+++ b/device.mk
@@ -579,7 +579,7 @@
 	vold.fstab
 
 #intialise PRODUCT_PACKAGES_DEBUG list for debug modules
-PRODUCT_PACKAGES_DEBUG := init.qcom.testscripts.sh
+PRODUCT_PACKAGES_DEBUG += init.qcom.testscripts.sh
 
 #Add init.qcom.test.rc to PRODUCT_PACKAGES_DEBUG list
 PRODUCT_PACKAGES_DEBUG += init.qcom.test.rc