package/dhcp: make S80dhcp-relay read a file from /etc/default/

The S80dhcp-relay init script has configuration variables like
INTERFACES whose contents have to be passed to the daemon. These
variables are initialized as empty strings, but some of them are not
allowed to be empty and there was no means of filling them apart from
creating a root FS overlay to overwrite these scripts.

This commit adds support for reading dhcrelay under /etc/default/ to
set these configuration variables.

[Thomas: adapt to patch only S80dhcp-relay, since S80dhcp-server has
already been changed by previous commits.]

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>