Rename is_unbundled_app_build and UNBUNDLED_APP_BUILD to TARGET_BUILD_APPS.

Change-Id: I79e6f60e009dad3a48709962347441e4ca0c337a
diff --git a/tools/Android.mk b/tools/Android.mk
index aea2785..fa9d0b7 100644
--- a/tools/Android.mk
+++ b/tools/Android.mk
@@ -16,7 +16,8 @@
 
 LOCAL_PATH := $(my-dir)
 
-ifeq ($(strip $(is_unbundled_app_build)),true)
+# If we're building only unbundled apps, this is the only tool we need.
+ifneq ($(TARGET_BUILD_APPS),)
 include $(LOCAL_PATH)/signapk/Android.mk
 else
 include $(call first-makefiles-under,$(LOCAL_PATH))