dhcp: S80dhcp-server: also source /etc/default/dhcpd.conf for INTERFACES

Similar to how we do it for dhcpd.service. The file should really have been
named /etc/default/dhcpd instead, but changing it now would break existing
systemd setups so leave it as it is.

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

package/dhcp/S80dhcp-server

@@ -7,6 +7,9 @@
 #       Separate multiple interfaces with spaces, e.g. "eth0 eth1".
 INTERFACES=""
 
+# Allow a few customizations from a config file
+test -r /etc/default/dhcpd.conf && . /etc/default/dhcpd.conf
+
 # Sanity checks
 test -f /usr/sbin/dhcpd || exit 0
 test -f /etc/dhcp/dhcpd.conf || exit 0