Hi there,
Is there are reason you are using 2.6 instead of 2.7? It should be BOARDS instead of BOARD, and you should specify the BOARDDIR for your custom board folder.
I highly recommend you use the PREBUILT and PYNQ_SDIST flags in your make call as well. If you are sticking with 2.6, the download link for the board agnostic PYNQ 2.6 image can be found in this forum post and the PYNQ 2.6 source distribution here.
then
make BOARDDIR=/full/path/to/your/boarddir PYNQ_SDIST=downloaded_sdist PREBUILT=downloaded_image BOARDS=ZCU102
If building more up to date 2.7 images, you can find the sdist in the release and the board agnostic rootfs image on the pynq website under boards.
Thanks
Shawn