Depending on what you want to do you can either delete the rd.gz from the .config file or the one extracted from the bootloader (https://mega.nz/#!rIk2BYYI!2a5_fZzyfqH7YUvmOyUXSniQeNe9nB9TkmI4qL680H0).
If you want to only compile kernel modules and use the original kernel then you can just delete rd.gz from the .config.
After the kernel module has been compiled you have to create a new init ram rd.gz and then add to the iso/usb and append the following argument to the kernel
initrd=rd.gz
, which it seems to work as a sort of overlay.
If you want to run your own kernel then you have to specify the path of your rd,gz in your .config file. In theory, this latter method should work, I got an error while installing the pat file. According to this post http://xpenology.com/forum/viewtopic.php?f=2&t=13076 the source code might not contain be complete.
Best
D.