dhcp: dhcpd.service: /etc/default/dhcpd.conf is optional

The environment file is for optional customization, so don't error out if it
isn't present.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dhcp/dhcpd.service

@@ -7,7 +7,7 @@ Type=forking
 PIDFile=/run/dhcpd.pid
 ExecStart=/usr/sbin/dhcpd -q -pf /run/dhcpd.pid $INTERFACES
 KillSignal=SIGINT
-EnvironmentFile=/etc/default/dhcpd.conf
+EnvironmentFile=-/etc/default/dhcpd.conf
 
 [Install]
 WantedBy=multi-user.target