dnsmasq: make tftp root if not existing

If there's a TFTP root directory configured, create it with mkdir -p
(which does not throw an error if the folder exists already)
before starting dnsmasq. This is useful for TFTP roots in /tmp, for example.

Originally submitted by nfw user aka Nathaniel Wesley Filardo

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>