tests/vm: Add vm-build-all/vm-clean-all in help text

Suggested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20180727083445.21436-1-famz@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/vm/Makefile.include

@@ -15,6 +15,9 @@ vm-test:
 	@echo "  vm-build-netbsd                 - Build QEMU in NetBSD VM"
 	@echo "  vm-build-openbsd                - Build QEMU in OpenBSD VM"
 	@echo "  vm-build-centos                 - Build QEMU in CentOS VM, with Docker"
+	@echo ""
+	@echo "  vm-build-all                    - Build QEMU in all VMs"
+	@echo "  vm-clean-all                    - Clean up VM images"
 
 vm-build-all: $(addprefix vm-build-, $(IMAGES))