Fixed the download URL for busybox-static.

Also implemented a proper check whether the download was succesful.
Merge pull request #169 from diederikdehaas/fix-busybox-download

Fixed the download URL for busybox-static.
Merge pull request #36 from debian-pi/v1.0.x

Pulling busybox fix from main line
Merge pull request #37 from Mausy5043/master

Pulling busy-box fix from `master`