uboot:mediatek:add default environment for mt7986
@AGnau : Update to corresponding Mediatek U-Boot version. Add hash to U-Boot tar-file in dl and to directory in build-dir to avoid that wrong version is used.
@AGnau : Update to corresponding Mediatek U-Boot version. Add hash to U-Boot tar-file in dl and to directory in build-dir to avoid that wrong version is used.