sudo apt install wget make gawk gcc bc bison flex xorriso libelf-dev libssl-dev

wget http://kernel.org/pub/linux/kernel/v4.x/linux-4.19.12.tar.xz
wget http://busybox.net/downloads/busybox-1.29.3.tar.bz2
wget http://kernel.org/pub/linux/utils/boot/syslinux/syslinux-6.03.tar.xz
mkdir isoimage
tar -xvf linux-4.19.12.tar.xz
tar -xvf busybox-1.29.3.tar.bz2
tar -xvf syslinux-6.03.tar.xz
cd busybox-1.29.3
make distclean defconfig
sed -i "s|.*CONFIG_STATIC.*|CONFIG_STATIC=y|" .config
make busybox install
cd _install
rm -f linuxrc
mkdir dev proc sys
echo '#!/bin/sh' > init
echo 'dmesg -n 1' >> init
echo 'mount -t devtmpfs none /dev' >> init
echo 'mount -t proc none /proc' >> init
echo 'mount -t sysfs none /sys' >> init
echo 'setsid cttyhack /bin/sh' >> init
chmod +x init
find . | cpio -R root:root -H newc -o | gzip > ../../isoimage/rootfs.gz
cd ../../linux-4.19.12
make mrproper defconfig bzImage
cp arch/x86/boot/bzImage ../isoimage/kernel.gz
cd ../isoimage
cp ../syslinux-6.03/bios/core/isolinux.bin .
cp ../syslinux-6.03/bios/com32/elflink/ldlinux/ldlinux.c32 .
echo 'default kernel.gz initrd=rootfs.gz' > ./isolinux.cfg
xorriso \
    -as mkisofs \
    -o ../minimal_linux_live.iso \
    -b isolinux.bin \
    -c boot.cat \
    -no-emul-boot \
    -boot-load-size 4 \
    -boot-info-table \
    ./
cd ..