You can make your kernel changes permanent with the following steps:

1. Create a default configuration from your .config file from the kernel source directory and a clean environment (not configured with a Yocto setup environment script) by running the following:

$ cd /opt/yocto/fsl-community-bsp/wandboard/tmp/work/wandboard-poky-linux-gnueabi/linux-wandboard/4.1-2.0.x-r0/build
$ make ARCH=arm savedefconfig  

2. Copy the defconfig file from your kernel build directory to your kernel recipe's defconfig file as follows:

$ cp defconfig /opt/yocto/fsl-community-bsp/sources/meta-bsp-custom/recipes-kernel/linux/linux-wandboard-4.1-2.0.x/wandboard-custom  

3. Alternatively, you can use BitBake from the build directory as follows:

$ bitbake -c savedefconfig virtual/kernel  

This also creates a defconfig file in the Linux kernel's build directory, which needs to be copied to your recipe.