initramfs-maker
There are 2 repositories under initramfs-maker topic.
Saruspete/initramfs_generator
Create your own minimal initramfs. No specific distribution required, just add the tools you want (from local os, chroot, image...) scripts or files, and it'll do the dependency checks and add them.
Ruulian/initramfs-toolkit
Toolkit that allows to extract and compress initramfs cpio, useful for Linux kernel exploitation.