Add scripts/single.sh to build individual non-multiplexed standalone commands.

Add scripts/single.sh to build individual non-multiplexed standalone commands.

Alas, you can't quite do this yet:

  make defconfig
  make
  for i in $(./toybox)
  do
    echo $i
    PREFIX=singles/ scripts/single.sh $i || break
  done

Because the OLDTOY() aliases for commands won't build without the base command.
And I can't just skip them because chown/chmod or mv/cp aren't the same thing.