Most exterior lights are designed to be mounted on top of light junction box. Such are readily available that can be surface mounted to your board.

You are not supposed to attach a plug/cord to a device that is meant to be hard-wired. It is your light, and you can do it, but you better make sure that you know what you are doing so it does not create a fire or shock hazard.

Actually there is no such prohibition.

The prohibition is that you can't use cords as a subsittute for fixed wiring, so if this board is a permanent attachment to the garage it's not legitimate.

There also is no ground wire on the light fixture.....only a white and black wire. How can I ground this then? Plus, the light is made where it looks light you cannot use a junction box.....it has to be attached directly to the side of the building. The wires would then be run through some conduit/tubing. Any advice....please!

If the fixture has no ground wire (which most do), some ground themselves through the mounting screws.

It is DESIGNED to mount to a box I can almost guarantee.
Commonly the box is mounted flush to the wall, but it can be (somewhat ugly) mounted on the surface.

You should not be running cordage through a conduit or "tube."

Why is it that you don't want to do this properly?

How about a photo or link to the light you are using?

I bought the light at Lowe's. It's a 42 watt dusk to dawn light. The brand is UTILITECH.......item/model # 135931

So reading the manual that I downloaded from the Lowes page

http://pdf.lowes.com/useandcareguides/016963568105_use.pdf

I see instructions for mounting WITH a junction box and WITHOUT a junction box.

I also see a ground screw in the light and the instructions say that the fixture MUST be connected to ground and point out the ground screw on the fixture.