tools/hotplug: Extend dhcpd conf, init and arg files search

tools/hotplug: Extend dhcpd conf, init and arg files search

Newer versions of the ISC dhcp server expect the dhcpd.conf file
to be located at /etc/dhcp directory.

Also, some distributions and Yocto based ones have these installation
paths by default: /etc/init.d/{isc-dhcp-server,dhcp-server} and
/etc/default/dhcp-server.

Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Wei Liu <wl@xen.org>