mkimage-arch: use official bootstrap to be distribution independent

The current mkimage-arch requires at least pacman to be installed
on main system. The official bootstrap introduced more than year ago
allows to make the script usable on non-Arch distributions.

The main downside of this method is theoretical outdatedness, as
bootstrap tarballs are created at the beginning of month. At the same
time generated image takes ca. 50MB less and should be more
upgradeproof.

Docker-DCO-1.1-Signed-off-by: Bartłomiej Piotrowski <b@bpiotrowski.pl> (github: Barthalion)