dhcp: update dhclient-script

The previous no-op dhclient-script is useless and in fact can lead
users to believe things will work.
dhclient-script is supposed to, among many other things, bring the
interface up for dhclient to pump an address, set the interface
address, set resolv.conf appropiately and set the default gateway.
It's a complex script since it has to deal with a lot of details so
let's just use the best fit which is the OpenWRT one since it uses the
"legacy" tools (ifconfig, route, ...) which can be easily provided by
busybox and/or net-tools.
There are newer and more feature-complete versions around but they
require full iproute2 ip and wouldn't fit as well with the busybox-only
approach.

License: likely GPLv2+
Source:
http://git.openwrt.org/?p=packages.git;a=blob;f=net/isc-dhcp/files/dhclient-script;h=4afebc0ad20ebac51c5baae5ed01c6713e3a0fd0;hb=HEAD



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>