kernel: Use the stock defconfig for /proc/config.gz

Userspace reads /proc/config.gz and spits out an error message after boot
finishes when it doesn't like the kernel's configuration. In order to
preserve our freedom to customize the kernel however we'd like, show
userspace the stock defconfig so that it never complains about our
kernel configuration.

Signed-off-by: Sultan Alsawaf <[email protected]>