In apps_only build dist a module only if it has a BUILT file.

This allows you to specify a PHONY target in TARGET_BUILD_APPS.

Change-Id: Idd813cd0dbdb14c705fbd3187596d4c64b99d045
diff --git a/core/main.mk b/core/main.mk
index 0b43efe..ba67189 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -819,7 +819,7 @@
   # For uninstallable modules such as static Java library, we have to dist the built file,
   # as <module_name>.<suffix>
   apps_only_dist_built_files := $(foreach m,$(unbundled_build_modules),$(if $(ALL_MODULES.$(m).INSTALLED),,\
-      $(ALL_MODULES.$(m).BUILT):$(m)$(suffix $(ALL_MODULES.$(m).BUILT))))
+      $(if $(ALL_MODULES.$(m).BUILT),$(ALL_MODULES.$(m).BUILT):$(m)$(suffix $(ALL_MODULES.$(m).BUILT)))))
   $(call dist-for-goals,apps_only, $(apps_only_dist_built_files))
 
   ifeq ($(EMMA_INSTRUMENT),true)