Hi Gaurav,
As promised, here is my script for building a super simple root filesystem with busybox. All it needs is for you to download a copy of the busybox source code and have the required cross compiler. You can pass the result to the QEMU with the -initrd parameter. You'll also need to pass "rdinit=/sbin/init" to the kernel when using these images.
I've also attached the images I've built for aarch64, arm and powerpc and the (really, really ugly) script I use for building and booting the kernel. My build and boot script probably isn't going to work for you out of the box. A lot of it is specific to my setup and has been hacked together over many years.
g.