But, I have not found how I can do this in Linux.. I have chosen the filesystem ROMFS in the .config file. Where and how the romfs tree is created and included to the kernel image?
We're using an embedded MIPS platform with busybox. We build the rootfs image (busybox build) and point to its location in the kernel menu config. Though I beleive that menu config is coming due to our vendor patched kernel. I'm not sure if this option is available in vanilla kernels. Doing this creates a cpio archive of whatever root dir we point to in the kernel config...which is then added to the kernel as a ramfs image.
This question concerns also all the necessary user applications like "init" and "sh".
If you have a busybox based system, then init is essentially the busybox shell. HTH, -mandeep