dnsmasq: add dhcp-script hook conditionally

dnsmasq: add dhcp-script hook conditionally

Commit b32689afd6a661339861086c669e15c936293cf8 added support for dhcp-script hook.
Adding dhcp-script config option results into two instances of dnsmasq being run
which triggered oom issues on platforms having low memory.

The dnsmasq dhcp-script config option will now only be added if at least one of the
dhcp, tftp, neigh hotplug dirs has a regular hotplug file or if the dhcpscript uci
config option is specified.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>