Generate an empty target if there is no packages

Generate an empty target even if there is no packages defined in the
source tree. Such case can happen when building in an unbundled branch.

Change-Id: Id8b94bc00557cd5ea2066859e440427474ced5f5

core/Makefile

@@ -234,6 +234,8 @@ $(APKCERTS_FILE):
 	         private_key=""' >> $@;,\
 	    echo 'name="$(p).apk" certificate="$(PACKAGES.$(p).CERTIFICATE)" \
 	         private_key="$(PACKAGES.$(p).PRIVATE_KEY)"' >> $@;))
+	# In case $(PACKAGES) is empty.
+	$(hide) touch $@
 
 .PHONY: apkcerts-list
 apkcerts-list: $(APKCERTS_FILE)