How to add root= details to the tiny-initrd.img?

I am using Debian 9 on QNAP TS-409 pro.

As initramfs-tools and dracut are giving result files too large (must be smaller than 4MB), I am trying to use tiny-initramfs.

For TS-409pro, there is no boot loader.

kernel is flash to /dev/mtdblock1
initrd is flashed to/dev/mtdblock2

So I need to include the root= parameter to the initrd.img file.

How can I do that? 