P1 mechanical: Add PEMs to tray

Currently, the board sits on standoffs which are manually attached to the tray by screws on the bottom side. This is not ideal when installing or removing the board - as you have screws at both ends of a standoff. I suggest adding some PEM (or similar) standoffs to the tray which the board could directly screw into, or at least some tapped holes to insert the standoffs.