# setup
apt update
apt-get install bzip2 git vim make gcc libncurses-dev flex bison bc cpio libelf-dev libssl-dev syslinux dosfstools

git clone --depth 1 https://github.com/torvalds/linux.git

cd linux

make menuconfig
# no change needed, just make sure 64bit is enabled

# check num of cores using nproc
make -j 4

mkdir /boot-files
cp arch/x86/boot/bzImage /boot-files/
cd ..

git clone --depth 1 https://git.busybox.net/busybox
cd busybox

# static build to keep it minimal
make menuconfig

make -j 4

mkdir /boot-files/initramfs
make CONFIG_PREFIX=/boot-files/initramfs install

cd /boot-files/initramfs/

echo -e '#!/bin/sh\n/bin/sh' > init
chmod +x init

rm linuxrc

find . | cpio -o -H newc > ../init.cpio
cd ..

dd if=/dev/zero of=boot bs=1M count=50

mkfs -t fat boot

syslinux boot

mkdir m
mount boot m
cp bzImage init.cpio m

umount m