add Backend.o to Makefile.legacy and only make the warning appear for the default target
add Backend.o to Makefile.legacy and only make the warning appear for the default target

crystalline/openc2e,ccdevnet/openc2e,crystalline/openc2e,bdonlan/openc2e,bdonlan/openc2e,ccdevnet/openc2e,ccdevnet/openc2e,ccdevnet/openc2e,crystalline/openc2e,bdonlan/openc2e,ccdevnet/openc2e,ccdevnet/openc2e,crystalline/openc2e,crystalline/openc2e,bdonlan/openc2e