ovs-sandbox: assign default value to ${MAKE}

ovs-sandbox: assign default value  to ${MAKE}

When run ovs-sandbox directly from the tutorial directory (the second method
in Tutorial.md), the following result show up:

$ ./ovs-sandbox -b /media/sda6/network/ovs/build
./ovs-sandbox: line 304: install-man: command not found

The reason is that ${MAKE} is not set in the situation.

So change ${MAKE} to ${MAKE-make} to resolve the issue.

GitHub-PR: #104
Signed-off-by: Qinghua Jin <qhjin_dev@163.com>
Signed-off-by: Russell Bryant <russell@ovn.org>