The latest firmware versions from D-Link will no longer accept the factory.bin format currently used by OpenWRT via the normal update procedure, so you'd have to flash via the emergency recovery, which only works with older browsers.
I'm trying to add support for the 'new style' image that can be flashed directly from the D-Link Web UI:
As I do not own DIR-859 myself, I could only verify image generation against the official firmware images available from D-Link, and it looks good so far.
However, before opening a pull request I would like to have someone confirm that the image is really accepted by the DIR-859 stock firmware:
openwrt-ath79-generic-dlink_dir-859-a1-squashfs-factory.bin
Is there any difference regarding versions A1 and A3? I could only find reva3 images on the German D-Link ftp, but they should be compatible to any rev A devices.