update-binary fails if tmp files already exist

[aggregate1166877]

I had a situation where a problem outside of the scope of this flasher meant I had to reflash the zip. Upon rerunning, I got the error "Recovery busybox setup failed. Aborting..."

Some hacking revealed that update-binary runs $bb ln -s $bb /tmp/anykernel/bin/$i;, which failed in my case because the soft links already existed. I resolved the issue by changing the ln command to $bb ln -sfn $bb /tmp/anykernel/bin/$i;.

I am unsure if the -fn options work with all versions of busybox, but I thought I'd mention this here in case it'd helpful.