debian/control: recommend busybox

  Error in context:
  | root@yanur:/src/githubclones/debos-recipes/debian/arm64/image-rpi3# debos debimage-rpi3.yaml
  | 2018/02/28 08:13:57 open failed: /bin/busybox - open /bin/busybox: no such file or directory

  When building rpi3 recipe:
  https://github.com/go-debos/debos-recipes/blob/master/debian/arm64/image-rpi3/debimage-rpi3.yaml



  Closes: #891714

Signed-off-by: Héctor Orón Martínez <zumbi@debian.org>