I followed the instructions at https://github.com/MediaTek-Labs/linkit-smart-7688-feed and https://forum.labs.mediatek.com/en/t/adventures-in-building-openwrt-for-linkit7688/258 to compile firmware and installed the new firmware on my LinkIt Smart 7688 Duo successfully.
root@mylinkit:~# uname -r
3.18.44