busybox

* Since busybox binary is unnecessary, deleted it from prerequisites.
* Changed so that not only links but /bin/busybox might be deleted.
* When double quoted PREFIX is defined by `.config' is used,
  `make uninstall' does not work correctly.
  When default PREFIX `pwd`/_install defined by Rules.mak is used,
  similarly it does not work correctly.
  Changed $$PREFIX into $(PREFIX) in order to fix this.
Patch by Hideki IWAMOTO