openvswitch: rework disabling docs build

This is needed since openvswitch 2.13 commit 2a97891eb23b
("Documentation: Work with sphinx-build for Python 3 also.")

The 4th patch was also reworked to serve as another guard

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

net/openvswitch/Makefile

@@ -257,7 +257,7 @@ CONFIGURE_VARS += \
 	ovs_cv_flake8=no \
 	ovs_cv_python3=$(PYTHON3) \
 	ovs_cv_python3_host=$(HOST_PYTHON3_BIN) \
-	ovs_cv_sphinx=no \
+	SPHINXBUILD=none \
 	KARCH=$(LINUX_KARCH) \
 
 ovs_intree_kmod_configs:=CONFIG_PACKAGE_kmod-openvswitch-intree