Hi sebtop,
I'm getting this error too. i dont think this way work anymore. So i try something else
I didn't change the kenrel-version.mk, i did something more simple like:
In my case, the kernel version is 3.18.84, so i just copy it:
cp ./feeds/linkit/mtk-sdk-wifi/wifi_binary/mt_wifi.ko_3.18.44 ./feeds/linkit/mtk-sdk-wifi/wifi_binary/mt_wifi.ko_3.18.84
cp ./feeds/linkit/mtk-sdk-wifi/wifi_binary/mt_wifi.ko_3.18.44_all ./feeds/linkit/mtk-sdk-wifi/wifi_binary/mt_wifi.ko_3.18.84_all
It tooks a lot of time, i'm still waiting 
I'll let you know if it works.