ifupdown.c: allow options to udhcpc to be configurable from .config

ifupdown.c: allow options to udhcpc to be configurable from .config

This allows more control over options that are passed to udhcpc
that are not configurable via /etc/network/interfaces.

Example: rather than quit when an initial lease isn't found,
you could use "--background" so that udhcpc keeps running and
continually attempts to get an ip.  That way, when you plug in
the ethernet after powering on the device, you'll still get an ip.