build-sys: make help could have 'modules' target

Available when configure --enable-modules.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180306161728.20890-1-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -1046,6 +1046,9 @@ include $(SRC_PATH)/tests/vm/Makefile.include
 help:
 	@echo  'Generic targets:'
 	@echo  '  all             - Build all'
+ifdef CONFIG_MODULES
+	@echo  '  modules         - Build all modules'
+endif
 	@echo  '  dir/file.o      - Build specified target only'
 	@echo  '  install         - Install QEMU, documentation and tools'
 	@echo  '  ctags/TAGS      - Generate tags file for editors'