This bundles all the board specific code in the mcu crate. There's much
to do though, so this is just a rough start and when it's all done we
should probably squash this.
Since the mcu crate dependencies require nightly and our CI builds with
stable, this removes the mcu backend from the default build and the ci.
* This requires passing the target through a thread-local (hence the corelib export
* Also fetch the background color and resize the item to match the draw target size
* The draw target is a generic and it just needs to support convertion from rgb888
