I've managed to compile a new openWRT firmware image by following the above instructions and installed it on my LinkIt Smart 7688 Duo.
root@mylinkit:/proc# cat version
Linux version 3.18.44 (wei-hsiung@weihsiung-ThinkPad-Edge-E420s) (gcc version 4.8.3 (OpenWrt/Linaro
GCC 4.8-2014.04 r49389) ) #1 Sat May 6 12:51:00 CST 2017
However, I received the below error messages when following the instructions at https://docs.labs.mediatek.com/resource/linkit-smart-7688/en/tutorials/file-storage/mounting-the-root-fs-on-an-sd-card.
root@mylinkit:~# opkg install kmod-usb-storage-extras
Installing kmod-usb-storage-extras (3.18.23-1) to root...
Multiple packages (kmod-usb-storage and kmod-usb-storage) providing same name marked HOLD or
PREFER. Using latest.
* satisfy_dependencies_for: Cannot satisfy the following dependencies for kmod-usb-storage-extras:
* kernel (= 3.18.23-1-32322c69cacfebf6e22f4e61cea9bebc) *
* opkg_install_cmd: Cannot install package kmod-usb-storage-extras.
Apparently, the problem has something to do with the mismatch of the kernel version between the installed one (3.18.44) and the one required by kmod-usb-storage-extras (3.18.23) and I think I need to change the configuration file before compiling the firmware (to use the kmod-usb-storage-extras and other packages that support kernel version 3.18.44). Could someone tell me how to do it? Thanks!!
After visiting https://downloads.openwrt.org/chaos_calmer/15.05.1/ramips/mt7688/packages/base/, I found that there is only one kmod-usb-storage-extras and it only supports kernel 3.18.23 (kmod-usb-storage-extras_3.18.23-1_ramips_24kec.ipk). Does it mean that I need to limit the kernel version to 3.18.23 and don't use 3.18.44?