mkarchiso: make sure to remove potentially preexisting files from $airootfs_dir before creating them with output redirection

mkarchiso creates "${airootfs_dir}/etc/machine-id" by using output redirection. If this file is an existing symlink, then the printf output would be written to the symlink target. It can be a big issue in case the symlink resolves to a path outside ${airootfs_dir}.

Fixes #121.