Sorry for bumping an old thread, but it's currently the top google search result for "openwrt CONFIG_DYNAMIC_DEBUG".
There's an option for this in the openwrt configuration which overrides the kernel configuration, so if you want to enable this you have to turn it on in
> Global build settings
> Kernel build options
> Compile the kernel with dynamic printk
Hopefully this saves someone else some time trying to figure out why you can't just put this option in your kernel config...