Previously multistrap was used to download and create an ubuntu rootfs;
but multistrap failed under docker build since it required binfmt
installed.

This technique of installing ubuntu rootfs is totally different method,
takes more time to build but works under the native runme.sh script and
under docker build.

The method idea is -
1. Build a simple rootfs with a kernel that can be booted via
qemu-system-aarch64.
2. Install an initi script called S99bootstrap-ubuntu.sh under
/etc/init.d.
3. Run a qemu-system-aarch64 virtual machine with 1GByte of system
memory and use cortex-a57 which is the closed to cortex-a72,
virtualization wise and has the buildroot rootf as initrd and the
destination ubuntu-core.ext4 partition as a drive.
4. The VM would boot the kernel, mount the rootfs and run the
S99bootstrap-ubuntu.sh script
5. The script would download an ubuntu base image and then 'apt install'
in a virtual chrooted environment the reset of the packages to make this
rootfs to be bootable.
6. Use 'reboot' in the VM which triggers the '-no-reboot' flag for qemu
that makes the vm exit; and thus installation is continued.
7. ubuntu-core.ext4 partition is now ready to be used further on for
imaging and installations.
Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>