busybox: udhcpc.script: cleanup

ifconfig up is a no-op if the device is already running, so let's just
do that unconditionally.

Systems might have multiple network devices, and perhaps run udhcpc on
another interface even when booted over nfs, so don't disable the
per-interface deconfig based on the global nfsroot= setting on the kernel
command line.

If you don't want udhcpc to mess with kernel level IP autoconfiguration
(E.G. for nfs boot), you should instead ensure udhcpc/ifup/ifplugd isn't
started for that interface.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>