Add beaglebone support.

To boot on the beaglebone, we need to install an MLO and patched u-boot, u-boot env, patched kernel and dtbs. These are the same files used by https://github.com/beagleboard/image-builder for the debian images released at http://beagleboard.org/latest-images. To my understanding, the patches are going to be merged into mainline u-boot / kernel, but I'm not sure about that status of that work.

I will also send a patch to Nick to add the beaglebone target to freedom-maker.