main/busybox: Fix resolv.conf update in udhcpc default.script

udhcpc updates /etc/resolv.conf by default (unless disabled by setting
"RESOLV_CONF=no" in /etc/udhcpc/udhcpc.conf).

The update is performed in the default script by writing to a temporary
resolv.conf file, which is then moved to /etc/resolv.conf via "mv". That
"mv" operation may become interactive, prompting "mv: overwrite
'/etc/resolv.conf'?", for example when /etc/resolv.conf is marked
immutable (chattr +i)

Since udhcpc is usually called during unattended boot, this may block
the device.

Change "mv" to "mv -f", which will not block (but may still fail to
update the file when /etc/resolv.conf is immutable).

Signed-off-by: Christian Kohlschütter <christian@kohlschutter.com>

Cheers,
Christian