Khadas tone board case

Basic standalone Khadas tone board case based of dimensions found [here](https://www.audiosciencereview.com/forum/index.php?threads/making-a-case-enclosure-for-the-khadas-tone-board.5054/). Not quite sure if things will fit as snugly as desired yet, but will update eventually when the board arrives and I do a few test fits, adjustments and measurements

12/5/2018: board arrived and after some test fitting, updated the dimensions and placements of the holes in the back plate for better a better