Support for unbundled app build

With this CL, run "make APP-{appname}" to build unbundled app in
the unbundled app src tree.
See http://b/issue?id=2667113

Change-Id: I1d753db795142508bc841ae66b4408220ab687c5
diff --git a/target/product/AndroidProducts.mk b/target/product/AndroidProducts.mk
index 60b4c9b..7798ea1 100644
--- a/target/product/AndroidProducts.mk
+++ b/target/product/AndroidProducts.mk
@@ -25,6 +25,12 @@
 # it includes.
 #
 
+ifeq ($(strip $(is_unbundled_app_build)),true)
+# An unbundled app build needs only generic.mk.
+PRODUCT_MAKEFILES := \
+    $(LOCAL_DIR)/core.mk \
+    $(LOCAL_DIR)/generic.mk
+else
 PRODUCT_MAKEFILES := \
     $(LOCAL_DIR)/core.mk \
     $(LOCAL_DIR)/generic.mk \
@@ -33,3 +39,4 @@
     $(LOCAL_DIR)/sdk.mk \
     $(LOCAL_DIR)/sim.mk \
     $(LOCAL_DIR)/large_emu_hw.mk
+endif