[udhcpd] Set subnet option so device is always reachable. Fixes: NB#780

Without the subnet option, some dhcp clients do not set the routing table
correctly. Which results in the device being unreachable unless a
corresponding route over the usb interface is set

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
[udhcpd] Also use subnet in init script. Fixes: NB#780

There is also a udhcpd.conf file written out by the init script.
This should also contain the subnet option to allow for correct
routing to be set.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
Merge pull request #39 from philippedeswert/741

[udhcpd] Set subnet option so device is always reachable. Fixes: NB#780