device-kobo-clara: split out downstream build as unmaintained

Near-mainline kernel is feature-complete since quite some time, so lets
use that as a default also to simplify boot process, since downstream kernel
depends on various blobs loaded into memory by the bootloader.
Get rid of quirks required in boot process for mainline kernels
also by using a near-mainline u-boot.
Keep Downstream kernel to be able to optimize/compare EPD operation