package/dhcp: systemd: support extra options

Add an OPTIONS configuration variable in order to make it possible to
pass custom extra options to dhcpd. This keeps the systemd support
consistent with the SysV init script.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>
Reviewed-by: "Maxime Hadjinlian" <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>