FriendlyELEC NanoPi K2 is a board powered by Amlogic S905 processor, just like ODROID-C2 board, so while only the Android image was available at launch, it was expected to also support Ubuntu or other Linux distribution shortly after. This was put in doubt by comments on the company’s forums claiming the board would not get Debian images, and only Android was supported.
One alternative would be Armbian, but right now they only have ODROID-C2 images for download, no other Amlogic S905 hardware platform is supported either through stable or experimental builds. One user did manage to run Armbian on K2 with balbes150 help, but I’m not sure what’s the status of those firmware. Balbes150 also have a list of image for Amlogic platform in Github, which may be adapted to most hardware by using your board’s device tree binary (DTB) file.