board/MynaPlayer odyssey: add default built-in barebox environment. #30

This adds the small overlay for the barebox bootloader's default environment. It mainly configures bootchooser properly as well as setting the auto boot timeout to 0. Bootchooser is configured to use the barebox state framework rather than non-volatile environment variables.

This PR should technically make it possible to get rid of the barebox environment once we hit something stable, since barebox doc is explicit about the persistent environment being intended for development purpose only, and any sane, in production system should rely on the state framework instead, along with a default, compiled-in environment.

Looks good to me! I compiled and booted but I get this message during boot:

Hit m for menu or any to stop autoboot:    0
ERROR: bootchooser: Cannot get state 'bootstate'
Nothing bootable found on 'bootchooser'
Nothing bootable found

I fixed this by running nv -r bootchooser.state_prefix, so maybe you need to remove the bootchooser.state_prefix file?

Ok that looks like my error, a clean rebuild fixes it. Merging!

board/MynaPlayer odyssey: add default built-in barebox environment.