In OpenWRT build, to change kernel config, you need to do the following:
1. make kernel_menuconfig CONFIG_TARGET=subtarget
This updates the .config file in build_dir/Linux-xxxxx
2. copy the file .config from the above directory to "target/Linux/your-board-name/config-3.14"
3. make V=99
The make command will copy the kernel config file from target/Linux to build_dir