This firmware file is the RTOS for the Spacemit K1x realtime CPU. The Spacemit firmware is based on the Thread Operating System RTOS, see https://www.rt-thread.io/ for an explanation. New esos.elf files can be downloaded from the Spacemit repo hosted on a Chinese GIT site, see https://gitee.com/spacemit-buildroot/buildroot-ext. One of the branches there also has a license file, reproduced here: https://gitee.com/spacemit-buildroot/buildroot-ext/blob/k1-bl-v2.2.y/board/spacemit/k1/target_overlay/lib/firmware/LICENSE.spacemit_esos